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I have tried to use Mongoose to send the list of all users as follows:
server.get('/usersList', function(req, res) {
var users = {};
User.find({}, function (err, user) {
users[user._id] = user;
});
res.send(users);
});
Of course, res.send(users); is going to send {}, which is not what I want. Is there a find alternative with slightly different semantics, where I could do the following?
server.get('/usersList', function(req, res) {
User.find({}, function (err, users) {
res.send(users);
});
});
Essentially, I want the callback to be executed only when all the users have been fetched from the database.
| Well, if you really want to return a mapping from _id to user, you could always do:
server.get('/usersList', function(req, res) {
User.find({}, function(err, users) {
var userMap = {};
users.forEach(function(user) {
userMap[user._id] = user;
});
res.send(userMap);
});
});
find() returns all matching documents in an array, so your last code snipped sends that array to the client.
| MongoDB | 14,103,615 | 126 |
I'm thinking of creating a multi-tenant app using MongoDB. I don't have any guesses in terms of how many tenants I'd have yet, but I would like to be able to scale into the thousands.
I can think of three strategies:
All tenants in the same collection, using tenant-specific fields for security
1 Collection per tenant in a single shared DB
1 Database per tenant
The voice in my head is suggesting that I go with option 2.
Thoughts and implications, anyone?
| I have the same problem to solve and also considering variants.
As I have years of experience creating SaaS multi-tenant applicatios I also was going to select the second option based on my previous experience with the relational databases.
While making my research I found this article on mongodb support site (way back added since it's gone):
https://web.archive.org/web/20140812091703/http://support.mongohq.com/use-cases/multi-tenant.html
The guys stated to avoid 2nd options at any cost, which as I understand is not particularly specific to mongodb. My impression is that this is applicable for most of the NoSQL dbs I researched (CoachDB, Cassandra, CouchBase Server, etc.) due to the specifics of the database design.
Collections (or buckets or however they call it in different DBs) are not the same thing as security schemas in RDBMS despite they behave as container for documents they are useless for applying good tenant separation. I couldn't find NoSQL database that can apply security restrictions based on collections.
Of course you can use mongodb role based security to restrict the access on database/server level. (http://docs.mongodb.org/manual/core/authorization/)
I would recommend 1st option when:
You have enough time and resources to deal with the complexity of the
design, implementation and testing of this scenario.
If you are not going to have much differences in structure and
functionality in the database for different tenants.
Your application design will allow tenants to make only minimal
customizations at runtime.
If you want to optimize space and minimize usage of hardware
resources.
If you are going to have thousands of tenants.
If you want to scale out fast and at good cost.
If you are NOT going to backup data based on tenants (keep separate
backups for each tenant). It is possible to do that even in this
scenario but the effort will be huge.
I would go for variant 3 if:
You are going to have small list of tenants (several hundred).
The specifics of the business requires you to be able to support big differences in the database structure for different tenants (e.g. integration with 3rd-party systems, import-export of data).
Your application design will allow customers (tenants) to make significant changes in the application runtime (adding modules, customizing the fields etc.).
If you have enough resources to scale out with new hardware nodes quickly.
If you are required to keep versions/backups of data per tenant. Also the restore will be easy.
There are legal/regulatory restrictions that forces you to keep different tenants in different databases (even data centers).
If you want to fully utilize the out-of-the-box security features of mongodb such as roles.
There are big differences in matter of size between tenants (you have many small tenants and few very large tenants).
If you post additional details about your application, perhaps I can give you more detailed advice.
| MongoDB | 2,748,825 | 126 |
The data type of the field is String. I would like to fetch the data where character length of field name is greater than 40.
I tried these queries but returning error.
1.
db.usercollection.find(
{$where: "(this.name.length > 40)"}
).limit(2);
output :error: {
"$err" : "TypeError: Cannot read property 'length' of undefined near '40)' ",
"code" : 16722
}
this is working in 2.4.9 But my version is 2.6.5
| For MongoDB 3.6 and newer:
The $expr operator allows the use of aggregation expressions within the query language, thus you can leverage the use of $strLenCP operator to check the length of the string as follows:
db.usercollection.find({
name: { $exists: true },
$expr: { $gt: [{ $strLenCP: '$name' }, 40] }
})
For MongoDB 3.4 and newer:
You can also use the aggregation framework with the $redact pipeline operator that allows you to proccess the logical condition with the $cond operator and uses the special operations $$KEEP to "keep" the document where the logical condition is true or $$PRUNE to "remove" the document where the condition was false.
This operation is similar to having a $project pipeline that selects the fields in the collection and creates a new field that holds the result from the logical condition query and then a subsequent $match, except that $redact uses a single pipeline stage which is more efficient.
As for the logical condition, there are String Aggregation Operators that you can use $strLenCP operator to check the length of the string. If the length is $gt a specified value, then this is a true match and the document is "kept". Otherwise it is "pruned" and discarded.
Consider running the following aggregate operation which demonstrates the above concept:
db.usercollection.aggregate([
{ $match: { name: { $exists: true } } },
{ $redact: {
$cond: [
{ $gt: [ { $strLenCP: "$name" }, 40] },
"$$KEEP",
"$$PRUNE"
]
} },
{ $limit: 2 }
])
If using $where, try your query without the enclosing brackets:
db.usercollection.find({ $where: "this.name.length > 40" }).limit(2);
A better query would be to to check for the field's existence and then check the length:
db.usercollection.find({ name: { $type: 2 }, $where: "this.name.length > 40" }).limit(2);
or:
db.usercollection.find({ name: { $exists: true }, $where: "this.name.length >
40" }).limit(2);
MongoDB evaluates non-$where query operations before $where expressions and non-$where query statements may use an index. A much better performance is to store the length of the string as another field and then you can index or search on it; applying $where will be much slower compared to that. It's recommended to use JavaScript expressions and the $where operator as a last resort when you can't structure the data in any other way, or when you are dealing with a
small subset of data.
A different and faster approach that avoids the use of the $where operator is the $regex operator. Consider the following pattern which searches for
db.usercollection.find({"name": {"$type": 2, "$regex": /^.{41,}$/}}).limit(2);
Note - From the docs:
If an index exists for the field, then MongoDB matches the regular
expression against the values in the index, which can be faster than a
collection scan. Further optimization can occur if the regular
expression is a “prefix expression”, which means that all potential
matches start with the same string. This allows MongoDB to construct a
“range” from that prefix and only match against those values from the
index that fall within that range.
A regular expression is a “prefix expression” if it starts with a
caret (^) or a left anchor (\A), followed by a string of simple
symbols. For example, the regex /^abc.*/ will be optimized by
matching only against the values from the index that start with abc.
Additionally, while /^a/, /^a.*/, and /^a.*$/ match equivalent
strings, they have different performance characteristics. All of these
expressions use an index if an appropriate index exists; however,
/^a.*/, and /^a.*$/ are slower. /^a/ can stop scanning after
matching the prefix.
| MongoDB | 29,577,713 | 125 |
I can use the SQL Like Operator using pymongo,
db.test.find({'c':{'$regex':'ttt'}})
But how can I use Not Like Operator?
I tried
db.test.find({'c':{'$not':{'$regex':'ttt'}})
but got error:
OperationFailure: $not cannot have a regex
| From the docs:
The $not operator does not support operations with the $regex
operator. Instead use // or in your driver interfaces, use your
language’s regular expression capability to create regular expression
objects. Consider the following example which uses the pattern match
expression //:
db.inventory.find( { item: { $not: /^p.*/ } } )
EDIT (@idbentley):
{$regex: 'ttt'} is generally equivalent to /ttt/ in mongodb, so your query would become:
db.test.find({c: {$not: /ttt/}}
EDIT2 (@KyungHoon Kim):
In python, below one works:
'c':{'$not':re.compile('ttt')}
| MongoDB | 20,175,122 | 125 |
Suppose I have two collections/schemas. One is the Users Schema with username and password fields, then, I have a Blogs Schema that has a reference to the Users Schema in the author field. If I use Mongoose to do something like
Blogs.findOne({...}).populate("user").exec()
I will have the Blog document and the user populated too, but how do I prevent Mongoose/MongoDB from returning the password field? The password field is hashed but it shouldn't be returned.
I know I can omit the password field and return the rest of the fields in a simple query, but how do I do that with populate. Also, is there any elegant way to do this?
Also, in some situations I do need to get the password field, like when the user wants to login or change the password.
| You can change the default behavior at the schema definition level using the select attribute of the field:
password: { type: String, select: false }
Then you can pull it in as needed in find and populate calls via field selection as '+password'. For example:
Users.findOne({_id: id}).select('+password').exec(...);
| MongoDB | 12,096,262 | 124 |
I'm using the MEAN stack and when I try to start the server using npm start, I get an exception saying that:
schema hasn't been registered for model 'Post'. Use mongoose.model(name, schema)
Here is my code inside /models/Posts.js:
var mongoose = require('mongoose');
var PostSchema = new mongoose.Schema({
title: String,
link: String,
upvotes: { type: Number, default: 0 },
comments: [{ type: mongoose.Schema.Types.ObjectId, ref: 'Comment' }]
});
mongoose.model('Post', PostSchema);
as I can see the schema should be registered for the model 'Post', but what is causing the exception to be thrown?
Edit: Here's the exception error:
/home/me/Documents/projects/personal/flapper-news/node_modules/mongoose/lib/index.js:323
throw new mongoose.Error.MissingSchemaError(name);
^
MissingSchemaError: Schema hasn't been registered for model "Post".
Use mongoose.model(name, schema)
and here's the app.js code with the mongoose initialization:
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/news');
require('./models/Posts');
require('./models/Comments');
before the line:
app.use('/', routes);
| It's not an issue with model export. I had the same issue.
The real issue is that require statements for the models
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/news');
require('./models/Posts');
require('./models/Comments');
were below the routes dependencies. Simply move the mongoDB dependencies above the routes dependencies. This is what it should look like:
// MongoDB
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/news');
require('./models/Posts');
require('./models/Comments');
var routes = require('./routes/index');
var users = require('./routes/users');
var app = express();
| MongoDB | 26,818,071 | 123 |
When using MongoDB's $in clause, does the order of the returned documents always correspond to the order of the array argument?
| As noted, the order of the arguments in the array of an $in clause does not reflect the order of how the documents are retrieved. That of course will be the natural order or by the selected index order as shown.
If you need to preserve this order, then you basically have two options.
So let's say that you were matching on the values of _id in your documents with an array that is going to be passed in to the $in as [ 4, 2, 8 ].
Approach using Aggregate
var list = [ 4, 2, 8 ];
db.collection.aggregate([
// Match the selected documents by "_id"
{ "$match": {
"_id": { "$in": [ 4, 2, 8 ] },
},
// Project a "weight" to each document
{ "$project": {
"weight": { "$cond": [
{ "$eq": [ "$_id", 4 ] },
1,
{ "$cond": [
{ "$eq": [ "$_id", 2 ] },
2,
3
]}
]}
}},
// Sort the results
{ "$sort": { "weight": 1 } }
])
So that would be the expanded form. What basically happens here is that just as the array of values is passed to $in you also construct a "nested" $cond statement to test the values and assign an appropriate weight. As that "weight" value reflects the order of the elements in the array, you can then pass that value to a sort stage in order to get your results in the required order.
Of course you actually "build" the pipeline statement in code, much like this:
var list = [ 4, 2, 8 ];
var stack = [];
for (var i = list.length - 1; i > 0; i--) {
var rec = {
"$cond": [
{ "$eq": [ "$_id", list[i-1] ] },
i
]
};
if ( stack.length == 0 ) {
rec["$cond"].push( i+1 );
} else {
var lval = stack.pop();
rec["$cond"].push( lval );
}
stack.push( rec );
}
var pipeline = [
{ "$match": { "_id": { "$in": list } }},
{ "$project": { "weight": stack[0] }},
{ "$sort": { "weight": 1 } }
];
db.collection.aggregate( pipeline );
Approach using mapReduce
Of course if that all seems to hefty for your sensibilities then you can do the same thing using mapReduce, which looks simpler but will likely run somewhat slower.
var list = [ 4, 2, 8 ];
db.collection.mapReduce(
function () {
var order = inputs.indexOf(this._id);
emit( order, { doc: this } );
},
function() {},
{
"out": { "inline": 1 },
"query": { "_id": { "$in": list } },
"scope": { "inputs": list } ,
"finalize": function (key, value) {
return value.doc;
}
}
)
And that basically relies on the emitted "key" values being in the "index order" of how they occur in the input array.
So those essentially are your ways of maintaining the order of a an input list to an $in condition where you already have that list in a determined order.
| MongoDB | 22,797,768 | 123 |
I am new(ish) to the whole javascript full stack applications, and completely new to Angular, so I was hoping somebody can put the record straight for me here.
Why would I need to use a templating framework like Jade or Handlebars when writing client side apps using AngularJS.
I should say that I have never used any of these templating frameworks either. So I am not familiar with the advantages completely. But when I look at Handlebars for example, it does many of the same things as I would do in Angular, such as looping etc.
As far as I can tell, it would make most sense to create the templates in Angular using proper HTML and then do all templating client side, and combine this with an API first approach using node and mongo for example.
The reason for this confusion is that a lot of the examples I find on GitHub make use of Jade, and it seems counter intuitive for me.
Please enlighten me, and set me straight. I would love to learn some best practices from people who know much more than I do.
Thanks
| I use Jade to generate templates consumed by AngularJS because I hate writing plain HTML. It looks something like:
.control-group(
ng-form
name='emailGroup'
ng-class='{"ng-error": emailGroup.$invalid}'
)
label.control-label Email
.controls
input(
type='email'
ng-model='user.email'
required
placeholder='[email protected]'
focus-on='focusEmail'
)
… which I think is a lot cleaner than plain HTML.
| MongoDB | 18,174,856 | 122 |
I know there are three different, popular types of non-sql databases.
Key/Value: Redis, Tokyo Cabinet, Memcached
ColumnFamily: Cassandra, HBase
Document: MongoDB, CouchDB
I have read long blogs about it without understanding so much.
I know relational databases and get the hang around document-based databases like MongoDB/CouchDB.
Could someone tell me what the major differences are between these and the 2 former on the list?
| The main differences are the data model and the querying capabilities.
Key-value stores
The first type is very simple and probably doesn't need any further explanation.
Data model: more than key-value stores
Although there is some debate on the correct name for databases such as Cassandra, I'd like to call them column-family stores. Although key-value pairs are an essential part of Cassandra, it's not limited to just that. It allows you to nest key-value pairs, so a key could refer to multiple sub-key-value pairs.
You cannot nest key-value pairs indefinitely though. You are limited to three levels (column families) or four levels of nesting (super-column families). In case the term column family doesn't ring a bell, see the WTF is a SuperColumn article, it's a good explanation of Cassandra's data model.
Document databases, such as CouchDB and MongoDB store entire documents in the form of JSON objects. You can think of these objects as nested key-value pairs. Unlike Cassandra, you can nest key-value pairs as much as you want. JSON also supports arrays and understands different data types, such as strings, numbers and boolean values.
Querying
I believe column-family stores can only be queried by key, or by writing map-reduce functions. You cannot query the values like you would in an SQL database. If your application needs more complex queries, your application will have to create and maintain indexes in order to access the desired data.
Document databases support queries by key and map-reduce functions as well, but also allow you to do basic queries by value, such as "Give me all users with more than 10 posts". Document databases are more flexible in this way.
| MongoDB | 3,554,169 | 121 |
To start the container, I am typing the following command:
sudo docker run -i -t -p 28000:27017 mongo:latest /usr/bin/mongod --smallfiles
But I want to open the shell in this container to type the mongo commands.
What command should I run to do the same?
| You can run the interactive mongo shell by running the following command:
docker run -it -p 28000:27017 --name mongoContainer mongo:latest mongosh
Otherwise, if your container is already running, you can use the exec command:
docker exec -it mongoContainer mongosh
| MongoDB | 32,944,729 | 121 |
I use NodeJS to insert documents in MongoDB. Using collection.insert I can insert a document into database like in this code:
// ...
collection.insert(objectToInsert, function(err){
if (err) return;
// Object inserted successfully.
var objectId; // = ???
});
// ...
How can I get the _id of inserted object?
Is there any way to get the _id without getting latest object inserted _id?
Supposing that in same time a lot of people access the database, I can't be sure that the latest id is the id of object inserted.
| A shorter way than using second parameter for the callback of collection.insert would be using objectToInsert._id that returns the _id (inside of the callback function, supposing it was a successful operation).
The Mongo driver for NodeJS appends the _id field to the original object reference, so it's easy to get the inserted id using the original object:
collection.insert(objectToInsert, function(err){
if (err) return;
// Object inserted successfully.
var objectId = objectToInsert._id; // this will return the id of object inserted
});
| MongoDB | 14,481,521 | 121 |
is possible in mongo db to select collection's documents like in SQL :
SELECT * FROM collection WHERE _id IN (1,2,3,4);
or if i have a _id array i must select one by one and then recompose the array/object of results?
| Easy :)
db.collection.find( { _id : { $in : [1,2,3,4] } } );
taken from: https://www.mongodb.com/docs/manual/reference/operator/query/in/#mongodb-query-op.-in
| MongoDB | 7,713,363 | 121 |
I have application using nodejs and mongodb. I have used mongoose for ODM.
Now i want to log all the queries that mongoose fire during the whole application.
How to log these?
| You can enable debug mode like so:
mongoose.set('debug', true);
or add your own debug callback:
mongoose.set('debug', function (coll, method, query, doc [, options]) {
//do your thing
});
This will log all executed collection methods and their arguments to the console.
| MongoDB | 18,762,264 | 120 |
I have a lot of mongodb documents in a collection of the form:
{
....
"URL":"www.abc.com/helloWorldt/..."
.....
}
I want to replace helloWorldt with helloWorld to get:
{
....
"URL":"www.abc.com/helloWorld/..."
.....
}
How can I achieve this for all documents in my collection?
| db.media.find({mediaContainer:"ContainerS3"}).forEach(function(e,i) {
e.url=e.url.replace("//a.n.com","//b.n.com");
db.media.save(e);
});
| MongoDB | 12,589,792 | 120 |
Would you recommend using a datetime or a timestamp field, and why (using MySQL)?
I'm working with PHP on the server side.
| Timestamps in MySQL are generally used to track changes to records, and are often updated every time the record is changed. If you want to store a specific value you should use a datetime field.
If you meant that you want to decide between using a UNIX timestamp or a native MySQL datetime field, go with the native DATETIME format. You can do calculations within MySQL that way
("SELECT DATE_ADD(my_datetime, INTERVAL 1 DAY)") and it is simple to change the format of the value to a UNIX timestamp ("SELECT UNIX_TIMESTAMP(my_datetime)") when you query the record if you want to operate on it with PHP.
Also, as of MySQL 8.0.19 the DATETIME supports time zone offsets, so there's even less reason to use TIMESTAMP now.
| MySQL | 409,286 | 3,180 |
I have a .sql file with an export from phpMyAdmin. I want to import it into a different server using the command line.
I have a Windows Server 2008 R2 installation. I placed the .sql file on the C drive, and I tried this command
database_name < file.sql
It is not working. I get syntax errors.
How can I import this file without a problem?
Do I need to create a database first?
| Try:
mysql -u username -p database_name < file.sql
Check MySQL Options.
Note 1: It is better to use the full path of the SQL file file.sql.
Note 2: Use -R and --triggers with mysqldump to keep the routines and triggers of the original database. They are not copied by default.
Note 3 You may have to create the (empty) database from MySQL if it doesn't exist already and the exported SQL doesn't contain CREATE DATABASE (exported with --no-create-db or -n option) before you can import it.
| MySQL | 17,666,249 | 2,785 |
If user input is inserted without modification into an SQL query, then the application becomes vulnerable to SQL injection, like in the following example:
$unsafe_variable = $_POST['user_input'];
mysql_query("INSERT INTO `table` (`column`) VALUES ('$unsafe_variable')");
That's because the user can input something like value'); DROP TABLE table;--, and the query becomes:
INSERT INTO `table` (`column`) VALUES('value'); DROP TABLE table;--')
What can be done to prevent this from happening?
| The correct way to avoid SQL injection attacks, no matter which database you use, is to separate the data from SQL, so that data stays data and will never be interpreted as commands by the SQL parser. It is possible to create an SQL statement with correctly formatted data parts, but if you don't fully understand the details, you should always use prepared statements and parameterized queries. These are SQL statements that are sent to and parsed by the database server separately from any parameters. This way it is impossible for an attacker to inject malicious SQL.
You basically have two options to achieve this:
Using PDO (for any supported database driver):
$stmt = $pdo->prepare('SELECT * FROM employees WHERE name = :name');
$stmt->execute([ 'name' => $name ]);
foreach ($stmt as $row) {
// Do something with $row
}
Using MySQLi (for MySQL):
Since PHP 8.2+ we can make use of execute_query() which prepares, binds parameters, and executes SQL statement in one method:
$result = $db->execute_query('SELECT * FROM employees WHERE name = ?', [$name]);
while ($row = $result->fetch_assoc()) {
// Do something with $row
}
Up to PHP8.1:
$stmt = $db->prepare('SELECT * FROM employees WHERE name = ?');
$stmt->bind_param('s', $name); // 's' specifies the variable type => 'string'
$stmt->execute();
$result = $stmt->get_result();
while ($row = $result->fetch_assoc()) {
// Do something with $row
}
If you're connecting to a database other than MySQL, there is a driver-specific second option that you can refer to (for example, pg_prepare() and pg_execute() for PostgreSQL). PDO is the universal option.
Correctly setting up the connection
PDO
Note that when using PDO to access a MySQL database real prepared statements are not used by default. To fix this you have to disable the emulation of prepared statements. An example of creating a connection using PDO is:
$dbConnection = new PDO('mysql:dbname=dbtest;host=127.0.0.1;charset=utf8mb4', 'user', 'password');
$dbConnection->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$dbConnection->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
In the above example, the error mode isn't strictly necessary, but it is advised to add it. This way PDO will inform you of all MySQL errors by means of throwing the PDOException.
What is mandatory, however, is the first setAttribute() line, which tells PDO to disable emulated prepared statements and use real prepared statements. This makes sure the statement and the values aren't parsed by PHP before sending it to the MySQL server (giving a possible attacker no chance to inject malicious SQL).
Although you can set the charset in the options of the constructor, it's important to note that 'older' versions of PHP (before 5.3.6) silently ignored the charset parameter in the DSN.
Mysqli
For mysqli we have to follow the same routine:
mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT); // error reporting
$dbConnection = new mysqli('127.0.0.1', 'username', 'password', 'test');
$dbConnection->set_charset('utf8mb4'); // charset
Explanation
The SQL statement you pass to prepare is parsed and compiled by the database server. By specifying parameters (either a ? or a named parameter like :name in the example above) you tell the database engine where you want to filter on. Then when you call execute, the prepared statement is combined with the parameter values you specify.
The important thing here is that the parameter values are combined with the compiled statement, not an SQL string. SQL injection works by tricking the script into including malicious strings when it creates SQL to send to the database. So by sending the actual SQL separately from the parameters, you limit the risk of ending up with something you didn't intend.
Any parameters you send when using a prepared statement will just be treated as strings (although the database engine may do some optimization so parameters may end up as numbers too, of course). In the example above, if the $name variable contains 'Sarah'; DELETE FROM employees the result would simply be a search for the string "'Sarah'; DELETE FROM employees", and you will not end up with an empty table.
Another benefit of using prepared statements is that if you execute the same statement many times in the same session it will only be parsed and compiled once, giving you some speed gains.
Oh, and since you asked about how to do it for an insert, here's an example (using PDO):
$preparedStatement = $db->prepare('INSERT INTO table (column) VALUES (:column)');
$preparedStatement->execute([ 'column' => $unsafeValue ]);
Can prepared statements be used for dynamic queries?
While you can still use prepared statements for the query parameters, the structure of the dynamic query itself cannot be parametrized and certain query features cannot be parametrized.
For these specific scenarios, the best thing to do is use a whitelist filter that restricts the possible values.
// Value whitelist
// $dir can only be 'DESC', otherwise it will be 'ASC'
if (empty($dir) || $dir !== 'DESC') {
$dir = 'ASC';
}
| MySQL | 60,174 | 2,769 |
What are the technical reasons for why one shouldn't use mysql_* functions? (e.g. mysql_query(), mysql_connect() or mysql_real_escape_string())?
Why should I use something else even if they work on my site?
If they don't work on my site, why do I get errors like
Warning: mysql_connect(): No such file or directory
| The MySQL extension:
Is not under active development
Is officially deprecated as of PHP 5.5 (released June 2013).
Has been removed entirely as of PHP 7.0 (released December 2015)
This means that as of 31 Dec 2018 it does not exist in any supported version of PHP. If you are using a version of PHP which supports it, you are using a version which doesn't get security problems fixed.
Lacks an OO interface
Doesn't support:
Non-blocking, asynchronous queries
Prepared statements or parameterized queries
Stored procedures
Multiple Statements
Transactions
The "new" password authentication method (on by default in MySQL 5.6; required in 5.7)
Any of the new functionality in MySQL 5.1 or later
Since it is deprecated, using it makes your code less future proof.
Lack of support for prepared statements is particularly important as they provide a clearer, less error-prone method of escaping and quoting external data than manually escaping it with a separate function call.
See the comparison of SQL extensions.
| MySQL | 12,859,942 | 2,722 |
I am trying to install version 1.2.2 of MySQL_python, using a fresh virtualenv created with the --no-site-packages option. The current version shown in PyPi is 1.2.3. Is there a way to install the older version? I have tried:
pip install MySQL_python==1.2.2
However, when installed, it still shows MySQL_python-1.2.3-py2.6.egg-info in the site packages. Is this a problem specific to this package, or am I doing something wrong?
| TL;DR:
Update as of 2022-12-28:
pip install --force-reinstall -v
For example: pip install --force-reinstall -v "MySQL_python==1.2.2"
What these options mean:
--force-reinstall is an option to reinstall all packages even if they are already up-to-date.
-v is for verbose. You can combine for even more verbosity (i.e. -vv) up to 3 times (e.g. --force-reinstall -vvv).
Thanks to @Peter for highlighting this (and it seems that the context of the question has broadened given the time when the question was first asked!), the documentation for Python discusses a caveat with using -I, in that it can break your installation if it was installed with a different package manager or if if your package is/was a different version.
Original answer:
pip install -Iv (i.e. pip install -Iv MySQL_python==1.2.2)
What these options mean:
-I stands for --ignore-installed which will ignore the installed packages, overwriting them.
-v is for verbose. You can combine for even more verbosity (i.e. -vv) up to 3 times (e.g. -Ivvv).
For more information, see pip install --help
First, I see two issues with what you're trying to do. Since you already have an installed version, you should either uninstall the current existing driver or use pip install -I MySQL_python==1.2.2
However, you'll soon find out that this doesn't work. If you look at pip's installation log, or if you do a pip install -Iv MySQL_python==1.2.2 you'll find that the PyPI URL link does not work for MySQL_python v1.2.2. You can verify this here: http://pypi.python.org/pypi/MySQL-python/1.2.2
The download link 404s and the fallback URL links are re-directing infinitely due to sourceforge.net's recent upgrade and PyPI's stale URL.
So to properly install the driver, you can follow these steps:
pip uninstall MySQL_python
pip install -Iv http://sourceforge.net/projects/mysql-python/files/mysql-python/1.2.2/MySQL-python-1.2.2.tar.gz/download
| MySQL | 5,226,311 | 1,977 |
I have this table for documents (simplified version here):
id
rev
content
1
1
...
2
1
...
1
2
...
1
3
...
How do I select one row per id and only the greatest rev?
With the above data, the result should contain two rows: [1, 3, ...] and [2, 1, ..]. I'm using MySQL.
Currently I use checks in the while loop to detect and over-write old revs from the resultset. But is this the only method to achieve the result? Isn't there a SQL solution?
| At first glance...
All you need is a GROUP BY clause with the MAX aggregate function:
SELECT id, MAX(rev)
FROM YourTable
GROUP BY id
It's never that simple, is it?
I just noticed you need the content column as well.
This is a very common question in SQL: find the whole data for the row with some max value in a column per some group identifier. I heard that a lot during my career. Actually, it was one the questions I answered in my current job's technical interview.
It is, actually, so common that Stack Overflow community has created a single tag just to deal with questions like that: greatest-n-per-group.
Basically, you have two approaches to solve that problem:
Joining with simple group-identifier, max-value-in-group Sub-query
In this approach, you first find the group-identifier, max-value-in-group (already solved above) in a sub-query. Then you join your table to the sub-query with equality on both group-identifier and max-value-in-group:
SELECT a.id, a.rev, a.contents
FROM YourTable a
INNER JOIN (
SELECT id, MAX(rev) rev
FROM YourTable
GROUP BY id
) b ON a.id = b.id AND a.rev = b.rev
Left Joining with self, tweaking join conditions and filters
In this approach, you left join the table with itself. Equality goes in the group-identifier. Then, 2 smart moves:
The second join condition is having left side value less than right value
When you do step 1, the row(s) that actually have the max value will have NULL in the right side (it's a LEFT JOIN, remember?). Then, we filter the joined result, showing only the rows where the right side is NULL.
So you end up with:
SELECT a.*
FROM YourTable a
LEFT OUTER JOIN YourTable b
ON a.id = b.id AND a.rev < b.rev
WHERE b.id IS NULL;
Conclusion
Both approaches bring the exact same result.
If you have two rows with max-value-in-group for group-identifier, both rows will be in the result in both approaches.
Both approaches are SQL ANSI compatible, thus, will work with your favorite RDBMS, regardless of its "flavor".
Both approaches are also performance friendly, however your mileage may vary (RDBMS, DB Structure, Indexes, etc.). So when you pick one approach over the other, benchmark. And make sure you pick the one which make most of sense to you.
| MySQL | 7,745,609 | 1,660 |
How can I reset the AUTO_INCREMENT of a field?
I want it to start counting from 1 again.
| You can reset the counter with:
ALTER TABLE tablename AUTO_INCREMENT = 1
InnoDB
For InnoDB you cannot set the auto_increment value lower or equal to the highest current index. (quote from ViralPatel):
Note that you cannot reset the counter to a value less than or equal
to any that have already been used. For MyISAM, if the value is less
than or equal to the maximum value currently in the AUTO_INCREMENT
column, the value is reset to the current maximum plus one. For
InnoDB, if the value is less than the current maximum value in the
column, no error occurs and the current sequence value is not changed.
Aria
In a table with the Aria storage engine the auto_increment value can be set to any value, even lower than that of the current maximum. The next insert however will use the next available value (max + 1) ignoring the set value. If set to a higher value it will continue to use and increment from that. The documentation is not particular clear on that but this was observed with Mariadb 10.11.3
See also
See How can I reset an MySQL AutoIncrement using a MAX value from another table? on how to dynamically get an acceptable value.
| MySQL | 8,923,114 | 1,640 |
Using MySQL, I can do something like:
SELECT hobbies FROM peoples_hobbies WHERE person_id = 5;
My Output:
shopping
fishing
coding
but instead I just want 1 row, 1 col:
Expected Output:
shopping, fishing, coding
The reason is that I'm selecting multiple values from multiple tables, and after all the joins I've got a lot more rows than I'd like.
I've looked for a function on MySQL Doc and it doesn't look like the CONCAT or CONCAT_WS functions accept result sets.
So does anyone here know how to do this?
| You can use GROUP_CONCAT:
SELECT person_id,
GROUP_CONCAT(hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;
As Ludwig stated in his comment, you can add the DISTINCT operator to avoid duplicates:
SELECT person_id,
GROUP_CONCAT(DISTINCT hobbies SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;
As Jan stated in their comment, you can also sort the values before imploding it using ORDER BY:
SELECT person_id,
GROUP_CONCAT(hobbies ORDER BY hobbies ASC SEPARATOR ', ')
FROM peoples_hobbies
GROUP BY person_id;
As Dag stated in his comment, there is a 1024 byte limit on the result. To solve this, run this query before your query:
SET group_concat_max_len = 2048;
Of course, you can change 2048 according to your needs. To calculate and assign the value:
SET group_concat_max_len = CAST(
(SELECT SUM(LENGTH(hobbies)) + COUNT(*) * LENGTH(', ')
FROM peoples_hobbies
GROUP BY person_id) AS UNSIGNED);
| MySQL | 276,927 | 1,488 |
Is there an easy way to run a MySQL query from the Linux command line and output the results in CSV format?
Here's what I'm doing now:
mysql -u uid -ppwd -D dbname << EOQ | sed -e 's/ /,/g' | tee list.csv
select id, concat("\"",name,"\"") as name
from students
EOQ
It gets messy when there are a lot of columns that need to be surrounded by quotes, or if there are quotes in the results that need to be escaped.
| From Save MySQL query results into a text or CSV file:
SELECT order_id,product_name,qty
FROM orders
WHERE foo = 'bar'
INTO OUTFILE '/var/lib/mysql-files/orders.csv'
FIELDS TERMINATED BY ','
ENCLOSED BY '"'
LINES TERMINATED BY '\n';
Note: That syntax may need to be reordered to
SELECT order_id,product_name,qty
INTO OUTFILE '/var/lib/mysql-files/orders.csv'
FIELDS TERMINATED BY ','
ENCLOSED BY '"'
LINES TERMINATED BY '\n'
FROM orders
WHERE foo = 'bar';
in more recent versions of MySQL.
Using this command, columns names will not be exported.
Also note that /var/lib/mysql-files/orders.csv will be on the server that is running MySQL. The user that the MySQL process is running under must have permissions to write to the directory chosen, or the command will fail.
If you want to write output to your local machine from a remote server (especially a hosted or virtualize machine such as Heroku or Amazon RDS), this solution is not suitable.
| MySQL | 356,578 | 1,437 |
I'm using the MySQL command-line utility and can navigate through a database. Now I need to see a list of user accounts. How can I do this?
I'm using MySQL version 5.4.1.
| Use this query:
SELECT User FROM mysql.user;
Which will output a table like this:
+-------+
| User |
+-------+
| root |
+-------+
| user2 |
+-------+
As Matthew Scharley points out in the comments on this answer, you can group by the User column if you'd only like to see unique usernames.
| MySQL | 1,135,245 | 1,417 |
Since MySQL doesn't seem to have any 'boolean' data type, which data type do you 'abuse' for storing true/false information in MySQL?
Especially in the context of writing and reading from/to a PHP script.
Over time I have used and seen several approaches:
tinyint, varchar fields containing the values 0/1,
varchar fields containing the strings '0'/'1' or 'true'/'false'
and finally enum Fields containing the two options 'true'/'false'.
None of the above seems optimal. I tend to prefer the tinyint 0/1 variant, since automatic type conversion in PHP gives me boolean values rather simply.
So which data type do you use? Is there a type designed for boolean values which I have overlooked? Do you see any advantages/disadvantages by using one type or another?
| For MySQL 5.0.3 and higher, you can use BIT. The manual says:
As of MySQL 5.0.3, the BIT data type is used to store bit-field
values. A type of BIT(M) enables storage of M-bit values. M can range
from 1 to 64.
Otherwise, according to the MySQL manual you can use BOOL or BOOLEAN, which are at the moment aliases of tinyint(1):
Bool, Boolean: These types are synonyms for TINYINT(1). A value of
zero is considered false. Non-zero
values are considered true.
MySQL also states that:
We intend to implement full boolean
type handling, in accordance with
standard SQL, in a future MySQL
release.
References: http://dev.mysql.com/doc/refman/5.5/en/numeric-type-overview.html
| MySQL | 289,727 | 1,398 |
I'm setting up a new server and want to support UTF-8 fully in my web application. I have tried this in the past on existing servers and always seem to end up having to fall back to ISO-8859-1.
Where exactly do I need to set the encoding/charsets? I'm aware that I need to configure Apache, MySQL, and PHP to do this — is there some standard checklist I can follow, or perhaps troubleshoot where the mismatches occur?
This is for a new Linux server, running MySQL 5, PHP, 5 and Apache 2.
| Data Storage:
Specify the utf8mb4 character set on all tables and text columns in your database. This makes MySQL physically store and retrieve values encoded natively in UTF-8. Note that MySQL will implicitly use utf8mb4 encoding if a utf8mb4_* collation is specified (without any explicit character set).
In older versions of MySQL (< 5.5.3), you'll unfortunately be forced to use simply utf8, which only supports a subset of Unicode characters. I wish I were kidding.
Data Access:
In your application code (e.g. PHP), in whatever DB access method you use, you'll need to set the connection charset to utf8mb4. This way, MySQL does no conversion from its native UTF-8 when it hands data off to your application and vice versa.
Some drivers provide their own mechanism for configuring the connection character set, which both updates its own internal state and informs MySQL of the encoding to be used on the connection—this is usually the preferred approach. In PHP:
If you're using the PDO abstraction layer with PHP ≥ 5.3.6, you can specify charset in the DSN:
$dbh = new PDO('mysql:charset=utf8mb4');
If you're using mysqli, you can call set_charset():
$mysqli->set_charset('utf8mb4'); // object oriented style
mysqli_set_charset($link, 'utf8mb4'); // procedural style
If you're stuck with plain mysql but happen to be running PHP ≥ 5.2.3, you can call mysql_set_charset.
If the driver does not provide its own mechanism for setting the connection character set, you may have to issue a query to tell MySQL how your application expects data on the connection to be encoded: SET NAMES 'utf8mb4'.
The same consideration regarding utf8mb4/utf8 applies as above.
Output:
UTF-8 should be set in the HTTP header, such as Content-Type: text/html; charset=utf-8. You can achieve that either by setting default_charset in php.ini (preferred), or manually using header() function.
If your application transmits text to other systems, they will also need to be informed of the character encoding. With web applications, the browser must be informed of the encoding in which data is sent (through HTTP response headers or HTML metadata).
When encoding the output using json_encode(), add JSON_UNESCAPED_UNICODE as a second parameter.
Input:
Browsers will submit data in the character set specified for the document, hence nothing particular has to be done on the input.
In case you have doubts about request encoding (in case it could be tampered with), you may verify every received string as being valid UTF-8 before you try to store it or use it anywhere. PHP's mb_check_encoding() does the trick, but you have to use it religiously. There's really no way around this, as malicious clients can submit data in whatever encoding they want, and I haven't found a trick to get PHP to do this for you reliably.
Other Code Considerations:
Obviously enough, all files you'll be serving (PHP, HTML, JavaScript, etc.) should be encoded in valid UTF-8.
You need to make sure that every time you process a UTF-8 string, you do so safely. This is, unfortunately, the hard part. You'll probably want to make extensive use of PHP's mbstring extension.
PHP's built-in string operations are not by default UTF-8 safe. There are some things you can safely do with normal PHP string operations (like concatenation), but for most things you should use the equivalent mbstring function.
To know what you're doing (read: not mess it up), you really need to know UTF-8 and how it works on the lowest possible level. Check out any of the links from utf8.com for some good resources to learn everything you need to know.
| MySQL | 279,170 | 1,358 |
There is a table messages that contains data as shown below:
Id Name Other_Columns
-------------------------
1 A A_data_1
2 A A_data_2
3 A A_data_3
4 B B_data_1
5 B B_data_2
6 C C_data_1
If I run a query select * from messages group by name, I will get the result as:
1 A A_data_1
4 B B_data_1
6 C C_data_1
What query will return the following result?
3 A A_data_3
5 B B_data_2
6 C C_data_1
That is, the last record in each group should be returned.
At present, this is the query that I use:
SELECT
*
FROM (SELECT
*
FROM messages
ORDER BY id DESC) AS x
GROUP BY name
But this looks highly inefficient. Any other ways to achieve the same result?
| MySQL 8.0 now supports windowing functions, like almost all popular SQL implementations. With this standard syntax, we can write greatest-n-per-group queries:
WITH ranked_messages AS (
SELECT m.*, ROW_NUMBER() OVER (PARTITION BY name ORDER BY id DESC) AS rn
FROM messages AS m
)
SELECT * FROM ranked_messages WHERE rn = 1;
This and other approaches to finding groupwise maximal rows are illustrated in the MySQL manual.
Below is the original answer I wrote for this question in 2009:
I write the solution this way:
SELECT m1.*
FROM messages m1 LEFT JOIN messages m2
ON (m1.name = m2.name AND m1.id < m2.id)
WHERE m2.id IS NULL;
Regarding performance, one solution or the other can be better, depending on the nature of your data. So you should test both queries and use the one that is better at performance given your database.
For example, I have a copy of the StackOverflow August data dump. I'll use that for benchmarking. There are 1,114,357 rows in the Posts table. This is running on MySQL 5.0.75 on my Macbook Pro 2.40GHz.
I'll write a query to find the most recent post for a given user ID (mine).
First using the technique shown by @Eric with the GROUP BY in a subquery:
SELECT p1.postid
FROM Posts p1
INNER JOIN (SELECT pi.owneruserid, MAX(pi.postid) AS maxpostid
FROM Posts pi GROUP BY pi.owneruserid) p2
ON (p1.postid = p2.maxpostid)
WHERE p1.owneruserid = 20860;
1 row in set (1 min 17.89 sec)
Even the EXPLAIN analysis takes over 16 seconds:
+----+-------------+------------+--------+----------------------------+-------------+---------+--------------+---------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+----------------------------+-------------+---------+--------------+---------+-------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 76756 | |
| 1 | PRIMARY | p1 | eq_ref | PRIMARY,PostId,OwnerUserId | PRIMARY | 8 | p2.maxpostid | 1 | Using where |
| 2 | DERIVED | pi | index | NULL | OwnerUserId | 8 | NULL | 1151268 | Using index |
+----+-------------+------------+--------+----------------------------+-------------+---------+--------------+---------+-------------+
3 rows in set (16.09 sec)
Now produce the same query result using my technique with LEFT JOIN:
SELECT p1.postid
FROM Posts p1 LEFT JOIN posts p2
ON (p1.owneruserid = p2.owneruserid AND p1.postid < p2.postid)
WHERE p2.postid IS NULL AND p1.owneruserid = 20860;
1 row in set (0.28 sec)
The EXPLAIN analysis shows that both tables are able to use their indexes:
+----+-------------+-------+------+----------------------------+-------------+---------+-------+------+--------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+----------------------------+-------------+---------+-------+------+--------------------------------------+
| 1 | SIMPLE | p1 | ref | OwnerUserId | OwnerUserId | 8 | const | 1384 | Using index |
| 1 | SIMPLE | p2 | ref | PRIMARY,PostId,OwnerUserId | OwnerUserId | 8 | const | 1384 | Using where; Using index; Not exists |
+----+-------------+-------+------+----------------------------+-------------+---------+-------+------+--------------------------------------+
2 rows in set (0.00 sec)
Here's the DDL for my Posts table:
CREATE TABLE `posts` (
`PostId` bigint(20) unsigned NOT NULL auto_increment,
`PostTypeId` bigint(20) unsigned NOT NULL,
`AcceptedAnswerId` bigint(20) unsigned default NULL,
`ParentId` bigint(20) unsigned default NULL,
`CreationDate` datetime NOT NULL,
`Score` int(11) NOT NULL default '0',
`ViewCount` int(11) NOT NULL default '0',
`Body` text NOT NULL,
`OwnerUserId` bigint(20) unsigned NOT NULL,
`OwnerDisplayName` varchar(40) default NULL,
`LastEditorUserId` bigint(20) unsigned default NULL,
`LastEditDate` datetime default NULL,
`LastActivityDate` datetime default NULL,
`Title` varchar(250) NOT NULL default '',
`Tags` varchar(150) NOT NULL default '',
`AnswerCount` int(11) NOT NULL default '0',
`CommentCount` int(11) NOT NULL default '0',
`FavoriteCount` int(11) NOT NULL default '0',
`ClosedDate` datetime default NULL,
PRIMARY KEY (`PostId`),
UNIQUE KEY `PostId` (`PostId`),
KEY `PostTypeId` (`PostTypeId`),
KEY `AcceptedAnswerId` (`AcceptedAnswerId`),
KEY `OwnerUserId` (`OwnerUserId`),
KEY `LastEditorUserId` (`LastEditorUserId`),
KEY `ParentId` (`ParentId`),
CONSTRAINT `posts_ibfk_1` FOREIGN KEY (`PostTypeId`) REFERENCES `posttypes` (`PostTypeId`)
) ENGINE=InnoDB;
Note to commenters: If you want another benchmark with a different version of MySQL, a different dataset, or different table design, feel free to do it yourself. I have shown the technique above. Stack Overflow is here to show you how to do software development work, not to do all the work for you.
| MySQL | 1,313,120 | 1,334 |
Between utf8_general_ci and utf8_unicode_ci, are there any differences in terms of performance?
| For those people still arriving at this question in 2020 or later, there are newer options that may be better than both of these. For example, utf8_unicode_520_ci.
All these collations are for the UTF-8 character encoding. The differences are in how text is sorted and compared.
_unicode_ci and _general_ci are two different sets of rules for sorting and comparing text according to the way we expect. Newer versions of MySQL introduce new sets of rules, too, such as _unicode_520_ci for equivalent rules based on Unicode 5.2, or the MySQL 8.x specific _0900_ai_ci for equivalent rules based on Unicode 9.0 (and with no equivalent _general_ci variant). People reading this now should probably use one of these newer collations instead of either _unicode_ci or _general_ci. The description of those older collations below is provided for interest only.
MySQL is currently transitioning away from an older, flawed UTF-8 implementation. For now, you need to use utf8mb4 instead of utf8 for the character encoding part, to ensure you are getting the fixed version. The flawed version remains for backward compatibility, though it is being deprecated.
Key differences
utf8mb4_unicode_ci is based on the official Unicode rules for universal sorting and comparison, which sorts accurately in a wide range of languages.
utf8mb4_general_ci is a simplified set of sorting rules which aims to do as well as it can while taking many short-cuts designed to improve speed. It does not follow the Unicode rules and will result in undesirable sorting or comparison in some situations, such as when using particular languages or characters.
On modern servers, this performance boost will be all but negligible. It was devised in a time when servers had a tiny fraction of the CPU performance of today's computers.
Benefits of utf8mb4_unicode_ci over utf8mb4_general_ci
utf8mb4_unicode_ci, which uses the Unicode rules for sorting and comparison, employs a fairly complex algorithm for correct sorting in a wide range of languages and when using a wide range of special characters. These rules need to take into account language-specific conventions; not everybody sorts their characters in what we would call 'alphabetical order'.
As far as Latin (ie "European") languages go, there is not much difference between the Unicode sorting and the simplified utf8mb4_general_ci sorting in MySQL, but there are still a few differences:
For examples, the Unicode collation sorts "ß" like "ss", and "Œ" like "OE" as people using those characters would normally want, whereas utf8mb4_general_ci sorts them as single characters (presumably like "s" and "e" respectively).
Some Unicode characters are defined as ignorable, which means they shouldn't count toward the sort order and the comparison should move on to the next character instead. utf8mb4_unicode_ci handles these properly.
In non-latin languages, such as Asian languages or languages with different alphabets, there may be a lot more differences between Unicode sorting and the simplified utf8mb4_general_ci sorting. The suitability of utf8mb4_general_ci will depend heavily on the language used. For some languages, it'll be quite inadequate.
What should you use?
There is almost certainly no reason to use utf8mb4_general_ci anymore, as we have left behind the point where CPU speed is low enough that the performance difference would be important. Your database will almost certainly be limited by other bottlenecks than this.
In the past, some people recommended to use utf8mb4_general_ci except when accurate sorting was going to be important enough to justify the performance cost. Today, that performance cost has all but disappeared, and developers are treating internationalization more seriously.
There's an argument to be made that if speed is more important to you than accuracy, you may as well not do any sorting at all. It's trivial to make an algorithm faster if you do not need it to be accurate. So, utf8mb4_general_ci is a compromise that's probably not needed for speed reasons and probably also not suitable for accuracy reasons.
One other thing I'll add is that even if you know your application only supports the English language, it may still need to deal with people's names, which can often contain characters used in other languages in which it is just as important to sort correctly. Using the Unicode rules for everything helps add peace of mind that the very smart Unicode people have worked very hard to make sorting work properly.
What the parts mean
Firstly, ci is for case-insensitive sorting and comparison. This means it's suitable for textual data, and case is not important. The other types of collation are cs (case-sensitive) for textual data where case is important, and bin, for where the encoding needs to match, bit for bit, which is suitable for fields which are really encoded binary data (including, for example, Base64). Case-sensitive sorting leads to some weird results and case-sensitive comparison can result in duplicate values differing only in letter case, so case-sensitive collations are falling out of favor for textual data - if case is significant to you, then otherwise ignorable punctuation and so on is probably also significant, and a binary collation might be more appropriate.
Next, unicode or general refers to the specific sorting and comparison rules - in particular, the way text is normalized or compared. There are many different sets of rules for the utf8mb4 character encoding, with unicode and general being two that attempt to work well in all possible languages rather than one specific one. The differences between these two sets of rules are the subject of this answer. Note that unicode uses rules from Unicode 4.0. Recent versions of MySQL and MariaDB add the rulesets unicode_520 using rules from Unicode 5.2, and MySQL 8.x adds 0900 (dropping the "unicode_" part) using rules from Unicode 9.0.
And lastly, utf8mb4 is of course the character encoding used internally. In this answer I'm talking only about Unicode based encodings.
| MySQL | 766,809 | 1,334 |
I want to update the column visited to give it the value 1. I tried this command in the SQL editor inside MySQL workbench:
UPDATE tablename SET columnname=1;
But I get an error that says:
You are using safe update mode and you tried to update a table without
a WHERE that uses a KEY column To disable safe mode, toggle the option
....
I followed the instructions, and I unchecked the safe update option from the Edit menu then Preferences then SQL Editor. But I still get the same error.
What is wrong? How can I update the value?
| It looks like your MySql session has the safe-updates option set. This means that you can't update or delete records without specifying a key (ex. primary key) in the where clause.
Try:
SET SQL_SAFE_UPDATES = 0;
Or you can modify your query to follow the rule (use primary key in where clause).
| MySQL | 11,448,068 | 1,210 |
I want to add a row to a database table, but if a row exists with the same unique key I want to update the row.
For example:
INSERT INTO table_name (ID, NAME, AGE) VALUES(1, "A", 19);
Let’s say the unique key is ID, and in my Database, there is a row with ID = 1. In that case, I want to update that row with these values. Normally this gives an error.
If I use INSERT IGNORE it will ignore the error, but it still won’t update.
| Use INSERT ... ON DUPLICATE KEY UPDATE
QUERY:
INSERT INTO table (id, name, age) VALUES(1, "A", 19) ON DUPLICATE KEY UPDATE
name="A", age=19
| MySQL | 4,205,181 | 1,169 |
I can run this query to get the sizes of all tables in a MySQL database:
show table status from myDatabaseName;
I would like some help in understanding the results. I am looking for tables with the largest sizes.
Which column should I look at?
| You can use this query to show the size of a table (although you need to substitute the variables first):
SELECT
table_name AS `Table`,
round(((data_length + index_length) / 1024 / 1024), 2) `Size in MB`
FROM information_schema.TABLES
WHERE table_schema = "$DB_NAME"
AND table_name = "$TABLE_NAME";
or this query to list the size of every table in every database, largest first:
SELECT
table_schema as `Database`,
table_name AS `Table`,
round(((data_length + index_length) / 1024 / 1024), 2) `Size in MB`
FROM information_schema.TABLES
ORDER BY (data_length + index_length) DESC;
| MySQL | 9,620,198 | 1,163 |
How do I quickly rename a MySQL database (change its schema name)?
Usually I just dump a database and re-import it with a new name. This is not an option for very big databases. Apparently RENAME {DATABASE | SCHEMA} db_name TO new_db_name; does bad things, exists only in a handful of versions, and is a bad idea overall.
This needs to work with InnoDB, which stores things very differently than MyISAM.
| For InnoDB, the following seems to work: create the new empty database, then rename each table in turn into the new database:
RENAME TABLE old_db.table TO new_db.table;
You will need to adjust the permissions after that.
For scripting in a shell, you can use either of the following:
mysql -u username -ppassword old_db -sNe 'show tables' | while read table; \
do mysql -u username -ppassword -sNe "rename table old_db.$table to new_db.$table"; done
OR
for table in `mysql -u root -ppassword -s -N -e "use old_db;show tables from old_db;"`; do mysql -u root -ppassword -s -N -e "use old_db;rename table old_db.$table to new_db.$table;"; done;
Notes:
There is no space between the option -p and the password. If your database has no password, remove the -u username -ppassword part.
If some table has a trigger, it cannot be moved to another database using above method (will result Trigger in wrong schema error). If that is the case, use a traditional way to clone a database and then drop the old one:
mysqldump old_db | mysql new_db
If you have stored procedures, you can copy them afterwards:
mysqldump -R old_db | mysql new_db
| MySQL | 67,093 | 1,156 |
I am an old-school MySQL user and have always preferred JOIN over sub-query. But nowadays everyone uses sub-query, and I hate it; I don't know why.
I lack the theoretical knowledge to judge for myself if there is any difference. Is a sub-query as good as a JOIN and therefore is there nothing to worry about?
| Sub-queries are the logically correct way to solve problems of the form, "Get facts from A, conditional on facts from B". In such instances, it makes more logical sense to stick B in a sub-query than to do a join. It is also safer, in a practical sense, since you don't have to be cautious about getting duplicated facts from A due to multiple matches against B.
Practically speaking, however, the answer usually comes down to performance. Some optimisers suck lemons when given a join vs a sub-query, and some suck lemons the other way, and this is optimiser-specific, DBMS-version-specific and query-specific.
Historically, explicit joins usually win, hence the established wisdom that joins are better, but optimisers are getting better all the time, and so I prefer to write queries first in a logically coherent way, and then restructure if performance constraints warrant this.
| MySQL | 2,577,174 | 1,123 |
For simplicity, assume all relevant fields are NOT NULL.
You can do:
SELECT
table1.this, table2.that, table2.somethingelse
FROM
table1, table2
WHERE
table1.foreignkey = table2.primarykey
AND (some other conditions)
Or else:
SELECT
table1.this, table2.that, table2.somethingelse
FROM
table1 INNER JOIN table2
ON table1.foreignkey = table2.primarykey
WHERE
(some other conditions)
Do these two work on the same way in MySQL?
| INNER JOIN is ANSI syntax that you should use.
It is generally considered more readable, especially when you join lots of tables.
It can also be easily replaced with an OUTER JOIN whenever a need arises.
The WHERE syntax is more relational model oriented.
A result of two tables JOINed is a cartesian product of the tables to which a filter is applied which selects only those rows with joining columns matching.
It's easier to see this with the WHERE syntax.
As for your example, in MySQL (and in SQL generally) these two queries are synonyms.
Also, note that MySQL also has a STRAIGHT_JOIN clause.
Using this clause, you can control the JOIN order: which table is scanned in the outer loop and which one is in the inner loop.
You cannot control this in MySQL using WHERE syntax.
| MySQL | 1,018,822 | 1,120 |
I started by googling and found the article How to write INSERT if NOT EXISTS queries in standard SQL which talks about mutex tables.
I have a table with ~14 million records. If I want to add more data in the same format, is there a way to ensure the record I want to insert does not already exist without using a pair of queries (i.e., one query to check and one to insert is the result set is empty)?
Does a unique constraint on a field guarantee the insert will fail if it's already there?
It seems that with merely a constraint, when I issue the insert via PHP, the script croaks.
| Use INSERT IGNORE INTO table.
There's also INSERT … ON DUPLICATE KEY UPDATE syntax, and you can find explanations in 13.2.6.2 INSERT ... ON DUPLICATE KEY UPDATE Statement.
Post from bogdan.org.ua according to Google's webcache:
18th October 2007
To start: as of the latest MySQL, syntax presented in the title is not
possible. But there are several very easy ways to accomplish what is
expected using existing functionality.
There are 3 possible solutions: using INSERT IGNORE, REPLACE, or
INSERT … ON DUPLICATE KEY UPDATE.
Imagine we have a table:
CREATE TABLE `transcripts` (
`ensembl_transcript_id` varchar(20) NOT NULL,
`transcript_chrom_start` int(10) unsigned NOT NULL,
`transcript_chrom_end` int(10) unsigned NOT NULL,
PRIMARY KEY (`ensembl_transcript_id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Now imagine that we have an automatic pipeline importing transcripts
meta-data from Ensembl, and that due to various reasons the pipeline
might be broken at any step of execution. Thus, we need to ensure two
things:
repeated executions of the pipeline will not destroy our
> database
repeated executions will not die due to ‘duplicate
> primary key’ errors.
Method 1: using REPLACE
It’s very simple:
REPLACE INTO `transcripts`
SET `ensembl_transcript_id` = 'ENSORGT00000000001',
`transcript_chrom_start` = 12345,
`transcript_chrom_end` = 12678;
If the record exists, it will be overwritten; if it does not yet
exist, it will be created. However, using this method isn’t efficient
for our case: we do not need to overwrite existing records, it’s fine
just to skip them.
Method 2: using INSERT IGNORE Also very simple:
INSERT IGNORE INTO `transcripts`
SET `ensembl_transcript_id` = 'ENSORGT00000000001',
`transcript_chrom_start` = 12345,
`transcript_chrom_end` = 12678;
Here, if the ‘ensembl_transcript_id’ is already present in the
database, it will be silently skipped (ignored). (To be more precise,
here’s a quote from MySQL reference manual: “If you use the IGNORE
keyword, errors that occur while executing the INSERT statement are
treated as warnings instead. For example, without IGNORE, a row that
duplicates an existing UNIQUE index or PRIMARY KEY value in the table
causes a duplicate-key error and the statement is aborted.”.) If the
record doesn’t yet exist, it will be created.
This second method has several potential weaknesses, including
non-abortion of the query in case any other problem occurs (see the
manual). Thus it should be used if previously tested without the
IGNORE keyword.
Method 3: using INSERT … ON DUPLICATE KEY UPDATE:
Third option is to use INSERT … ON DUPLICATE KEY UPDATE
syntax, and in the UPDATE part just do nothing do some meaningless
(empty) operation, like calculating 0+0 (Geoffray suggests doing the
id=id assignment for the MySQL optimization engine to ignore this
operation). Advantage of this method is that it only ignores duplicate
key events, and still aborts on other errors.
As a final notice: this post was inspired by Xaprb. I’d also advise to
consult his other post on writing flexible SQL queries.
| MySQL | 1,361,340 | 1,095 |
I have a table:
table votes (
id,
user,
email,
address,
primary key(id),
);
How can I make the columns user, email, address unique - i.e., ensure that there isn't any pair of rows that has identical values for all three columns?
| To add a unique constraint, you need to use two components:
ALTER TABLE - to change the table schema and,
ADD UNIQUE - to add the unique constraint.
You then can define your new unique key with the format 'name'('column1', 'column2'...)
So for your particular issue, you could use this command:
ALTER TABLE `votes` ADD UNIQUE `unique_index`(`user`, `email`, `address`);
| MySQL | 635,937 | 1,049 |
I have 2-3 different column names that I want to look up in the entire database and list out all tables which have those columns. Is there any easy script?
| To get all tables with columns columnA or ColumnB in the database YourDatabase:
SELECT DISTINCT TABLE_NAME
FROM INFORMATION_SCHEMA.COLUMNS
WHERE COLUMN_NAME IN ('columnA','ColumnB')
AND TABLE_SCHEMA='YourDatabase';
| MySQL | 193,780 | 1,048 |
How do you set a default value for a MySQL Datetime column?
In SQL Server it's getdate(). What is the equivalant for MySQL? I'm using MySQL 5.x if that is a factor.
| IMPORTANT EDIT:
It is now possible to achieve this with DATETIME fields since MySQL 5.6.5, take a look at the other post below...
Previous versions can't do that with DATETIME...
But you can do it with TIMESTAMP:
mysql> create table test (str varchar(32), ts TIMESTAMP DEFAULT CURRENT_TIMESTAMP);
Query OK, 0 rows affected (0.00 sec)
mysql> desc test;
+-------+-------------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+-------------------+-------+
| str | varchar(32) | YES | | NULL | |
| ts | timestamp | NO | | CURRENT_TIMESTAMP | |
+-------+-------------+------+-----+-------------------+-------+
2 rows in set (0.00 sec)
mysql> insert into test (str) values ("demo");
Query OK, 1 row affected (0.00 sec)
mysql> select * from test;
+------+---------------------+
| str | ts |
+------+---------------------+
| demo | 2008-10-03 22:59:52 |
+------+---------------------+
1 row in set (0.00 sec)
mysql>
CAVEAT: IF you define a column with CURRENT_TIMESTAMP ON as default, you will need to ALWAYS specify a value for this column or the value will automatically reset itself to "now()" on update. This means that if you do not want the value to change, your UPDATE statement must contain "[your column name] = [your column name]" (or some other value) or the value will become "now()". Weird, but true. I am using 5.5.56-MariaDB
| MySQL | 168,736 | 1,041 |
I am trying to select data from a MySQL table, but I get one of the following error messages:
mysql_fetch_array() expects parameter 1 to be resource, boolean given
This is my code:
$username = $_POST['username'];
$password = $_POST['password'];
$result = mysql_query('SELECT * FROM Users WHERE UserName LIKE $username');
while($row = mysql_fetch_array($result)) {
echo $row['FirstName'];
}
| A query may fail for various reasons in which case both the mysql_* and the mysqli extension will return false from their respective query functions/methods. You need to test for that error condition and handle it accordingly.
mysql_ extension:
NOTE The mysql_ functions are deprecated and have been removed in php version 7.
Check $result before passing it to mysql_fetch_array. You'll find that it's false because the query failed. See the [mysql_query][1] documentation for possible return values and suggestions for how to deal with them.
$username = mysql_real_escape_string($_POST['username']);
$password = $_POST['password'];
$result = mysql_query("SELECT * FROM Users WHERE UserName LIKE '$username'");
if($result === FALSE) {
trigger_error(mysql_error(), E_USER_ERROR);
}
while($row = mysql_fetch_array($result))
{
echo $row['FirstName'];
}
| MySQL | 2,973,202 | 1,005 |
While executing an INSERT statement with many rows, I want to skip duplicate entries that would otherwise cause failure. After some research, my options appear to be the use of either:
ON DUPLICATE KEY UPDATE which implies an unnecessary update at some cost, or
INSERT IGNORE implies an invitation for other kinds of failure to slip in unannounced.
Am I right in these assumptions? What's the best way to simply skip the rows that might cause duplicates and just continue on to the other rows?
| I would recommend using INSERT...ON DUPLICATE KEY UPDATE.
If you use INSERT IGNORE, then the row won't actually be inserted if it results in a duplicate key. But the statement won't generate an error. It generates a warning instead. These cases include:
Inserting a duplicate key in columns with PRIMARY KEY or UNIQUE constraints.
Inserting a NULL into a column with a NOT NULL constraint.
Inserting a row to a partitioned table, but the values you insert don't map to a partition.
If you use REPLACE, MySQL actually does a DELETE followed by an INSERT internally, which has some unexpected side effects:
A new auto-increment ID is allocated.
Dependent rows with foreign keys may be deleted (if you use cascading foreign keys) or else prevent the REPLACE.
Triggers that fire on DELETE are executed unnecessarily.
Side effects are propagated to replicas too.
correction: both REPLACE and INSERT...ON DUPLICATE KEY UPDATE are non-standard, proprietary inventions specific to MySQL. ANSI SQL 2003 defines a MERGE statement that can solve the same need (and more), but MySQL does not support the MERGE statement.
A user tried to edit this post (the edit was rejected by moderators). The edit tried to add a claim that INSERT...ON DUPLICATE KEY UPDATE causes a new auto-increment id to be allocated. It's true that the new id is generated, but it is not used in the changed row.
See demonstration below, tested with Percona Server 5.5.28. The configuration variable innodb_autoinc_lock_mode=1 (the default):
mysql> create table foo (id serial primary key, u int, unique key (u));
mysql> insert into foo (u) values (10);
mysql> select * from foo;
+----+------+
| id | u |
+----+------+
| 1 | 10 |
+----+------+
mysql> show create table foo\G
CREATE TABLE `foo` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`u` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `u` (`u`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=latin1
mysql> insert into foo (u) values (10) on duplicate key update u = 20;
mysql> select * from foo;
+----+------+
| id | u |
+----+------+
| 1 | 20 |
+----+------+
mysql> show create table foo\G
CREATE TABLE `foo` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`u` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `u` (`u`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1
The above demonstrates that the IODKU statement detects the duplicate, and invokes the update to change the value of u. Note the AUTO_INCREMENT=3 indicates an id was generated, but not used in the row.
Whereas REPLACE does delete the original row and inserts a new row, generating and storing a new auto-increment id:
mysql> select * from foo;
+----+------+
| id | u |
+----+------+
| 1 | 20 |
+----+------+
mysql> replace into foo (u) values (20);
mysql> select * from foo;
+----+------+
| id | u |
+----+------+
| 3 | 20 |
+----+------+
| MySQL | 548,541 | 959 |
Per the MySQL docs, there are four TEXT types:
TINYTEXT
TEXT
MEDIUMTEXT
LONGTEXT
What is the maximum length that I can store in a column of each data type assuming the character encoding is UTF-8?
| From the documentation (MySQL 8) :
Type | Maximum length
-----------+-------------------------------------
TINYTEXT | 255 (2 8−1) bytes
TEXT | 65,535 (216−1) bytes = 64 KiB
MEDIUMTEXT | 16,777,215 (224−1) bytes = 16 MiB
LONGTEXT | 4,294,967,295 (232−1) bytes = 4 GiB
Note that the number of characters that can be stored in your column will depend on the character encoding.
| MySQL | 13,932,750 | 951 |
I have a table with a varchar column, and I would like to find all the records that have duplicate values in this column. What is the best query I can use to find the duplicates?
| Do a SELECT with a GROUP BY clause. Let's say name is the column you want to find duplicates in:
SELECT name, COUNT(*) c FROM table GROUP BY name HAVING c > 1;
This will return a result with the name value in the first column, and a count of how many times that value appears in the second.
| MySQL | 688,549 | 948 |
I've got a messages table in MySQL which records messages between users. Apart from the typical ids and message types (all integer types) I need to save the actual message text as either VARCHAR or TEXT. I'm setting a front-end limit of 3000 characters which means the messages would never be inserted into the db as longer than this.
Is there a rationale for going with either VARCHAR(3000) or TEXT? There's something about just writing VARCHAR(3000) that feels somewhat counter-intuitive. I've been through other similar posts on Stack Overflow but would be good to get views specific to this type of common message storing.
|
TEXT and BLOB may by stored off the table with the table just having a pointer to the location of the actual storage. Where it is stored depends on lots of things like data size, columns size, row_format, and MySQL version.
VARCHAR is stored inline with the table. VARCHAR is faster when the size is reasonable, the tradeoff of which would be faster depends upon your data and your hardware, you'd want to benchmark a real-world scenario with your data.
| MySQL | 2,023,481 | 914 |
I have a table of player performance:
CREATE TABLE TopTen (
id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
home INT UNSIGNED NOT NULL,
`datetime`DATETIME NOT NULL,
player VARCHAR(6) NOT NULL,
resource INT NOT NULL
);
What query will return the rows for each distinct home holding its maximum value of datetime? In other words, how can I filter by the maximum datetime (grouped by home) and still include other non-grouped, non-aggregate columns (such as player) in the result?
For this sample data:
INSERT INTO TopTen
(id, home, `datetime`, player, resource)
VALUES
(1, 10, '04/03/2009', 'john', 399),
(2, 11, '04/03/2009', 'juliet', 244),
(5, 12, '04/03/2009', 'borat', 555),
(3, 10, '03/03/2009', 'john', 300),
(4, 11, '03/03/2009', 'juliet', 200),
(6, 12, '03/03/2009', 'borat', 500),
(7, 13, '24/12/2008', 'borat', 600),
(8, 13, '01/01/2009', 'borat', 700)
;
the result should be:
id
home
datetime
player
resource
1
10
04/03/2009
john
399
2
11
04/03/2009
juliet
244
5
12
04/03/2009
borat
555
8
13
01/01/2009
borat
700
I tried a subquery getting the maximum datetime for each home:
-- 1 ..by the MySQL manual:
SELECT DISTINCT
home,
id,
datetime AS dt,
player,
resource
FROM TopTen t1
WHERE `datetime` = (SELECT
MAX(t2.datetime)
FROM TopTen t2
GROUP BY home)
GROUP BY `datetime`
ORDER BY `datetime` DESC
The result-set has 130 rows although database holds 187, indicating the result includes some duplicates of home.
Then I tried joining to a subquery that gets the maximum datetime for each row id:
-- 2 ..join
SELECT
s1.id,
s1.home,
s1.datetime,
s1.player,
s1.resource
FROM TopTen s1
JOIN (SELECT
id,
MAX(`datetime`) AS dt
FROM TopTen
GROUP BY id) AS s2
ON s1.id = s2.id
ORDER BY `datetime`
Nope. Gives all the records.
I tried various exotic queries, each with various results, but nothing that got me any closer to solving this problem.
| You are so close! All you need to do is select BOTH the home and its max date time, then join back to the topten table on BOTH fields:
SELECT tt.*
FROM topten tt
INNER JOIN
(SELECT home, MAX(datetime) AS MaxDateTime
FROM topten
GROUP BY home) groupedtt
ON tt.home = groupedtt.home
AND tt.datetime = groupedtt.MaxDateTime
| MySQL | 612,231 | 912 |
I want to do a full outer join in MySQL. Is this possible? Is a full outer join supported by MySQL?
| You don't have full joins in MySQL, but you can sure emulate them.
For a code sample transcribed from this Stack Overflow question you have:
With two tables t1, t2:
SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
The query above works for special cases where a full outer join operation would not produce any duplicate rows. The query above depends on the UNION set operator to remove duplicate rows introduced by the query pattern. We can avoid introducing duplicate rows by using an anti-join pattern for the second query, and then use a UNION ALL set operator to combine the two sets. In the more general case, where a full outer join would return duplicate rows, we can do this:
SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
UNION ALL
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
WHERE t1.id IS NULL
| MySQL | 4,796,872 | 883 |
I accidentally enabled ONLY_FULL_GROUP_BY mode like this:
SET sql_mode = 'ONLY_FULL_GROUP_BY';
How do I disable it?
| Solution 1:
Remove ONLY_FULL_GROUP_BY from mysql console
mysql > SET GLOBAL sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));
you can read more here
Be aware that this setting is NOT persistent across restarts, then use
mysql > SET PERSIST sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));
Solution 2: Remove ONLY_FULL_GROUP_BY from phpmyadmin
Open phpmyadmin & select localhost
Click on menu Variables & scroll down for sql mode
Click on edit button to change the values & remove ONLY_FULL_GROUP_BY & click on save.
| MySQL | 23,921,117 | 882 |
This should be dead simple, but I cannot get it to work for the life of me.
I'm just trying to connect remotely to my MySQL server.
Connecting as:
mysql -u root -h localhost -p
works fine, but trying:
mysql -u root -h 'any ip address here' -p
fails with the error:
ERROR 1130 (00000): Host 'xxx.xx.xxx.xxx' is not allowed to connect to this MySQL server
In the mysql.user table, there is exactly the same entry for user 'root' with host 'localhost' as another with host '%'.
I'm at my wits' end and have no idea how to proceed.
Any ideas are welcome.
| Possibly a security precaution. You could try adding a new administrator account:
mysql> CREATE USER 'monty'@'localhost' IDENTIFIED BY 'some_pass';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'localhost'
-> WITH GRANT OPTION;
mysql> CREATE USER 'monty'@'%' IDENTIFIED BY 'some_pass';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'%'
-> WITH GRANT OPTION;
Although as Pascal and others have noted it's not a great idea to have a user with this kind of access open to any IP. If you need an administrative user, use root, and leave it on localhost. For any other action specify exactly the privileges you need and limit the accessibility of the user as Pascal has suggest below.
Edit:
From the MySQL FAQ:
If you cannot figure out why you get
Access denied, remove from the user
table all entries that have Host
values containing wildcards (entries
that contain '%' or '_' characters). A
very common error is to insert a new
entry with Host='%' and
User='some_user', thinking that this
allows you to specify localhost to
connect from the same machine. The
reason that this does not work is that
the default privileges include an
entry with Host='localhost' and
User=''. Because that entry has a Host
value 'localhost' that is more
specific than '%', it is used in
preference to the new entry when
connecting from localhost! The correct
procedure is to insert a second entry
with Host='localhost' and
User='some_user', or to delete the
entry with Host='localhost' and
User=''. After deleting the entry,
remember to issue a FLUSH PRIVILEGES
statement to reload the grant tables.
See also Section 5.4.4, “Access
Control, Stage 1: Connection
Verification”.
| MySQL | 1,559,955 | 881 |
Why doesn't a TRUNCATE on mygroup work?
Even though I have ON DELETE CASCADE SET I get:
ERROR 1701 (42000): Cannot truncate a table referenced in a foreign key constraint (mytest.instance, CONSTRAINT instance_ibfk_1 FOREIGN KEY (GroupID) REFERENCES mytest.mygroup (ID))
drop database mytest;
create database mytest;
use mytest;
CREATE TABLE mygroup (
ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY
) ENGINE=InnoDB;
CREATE TABLE instance (
ID INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
GroupID INT NOT NULL,
DateTime DATETIME DEFAULT NULL,
FOREIGN KEY (GroupID) REFERENCES mygroup(ID) ON DELETE CASCADE,
UNIQUE(GroupID)
) ENGINE=InnoDB;
| Yes you can:
SET FOREIGN_KEY_CHECKS = 0;
TRUNCATE table1;
TRUNCATE table2;
SET FOREIGN_KEY_CHECKS = 1;
With these statements, you risk letting in rows into your tables that do not adhere to the FOREIGN KEY constraints.
| MySQL | 5,452,760 | 865 |
I have a table story_category in my database with corrupt entries. The next query returns the corrupt entries:
SELECT *
FROM story_category
WHERE category_id NOT IN (
SELECT DISTINCT category.id
FROM category INNER JOIN
story_category ON category_id=category.id);
I tried to delete them executing:
DELETE FROM story_category
WHERE category_id NOT IN (
SELECT DISTINCT category.id
FROM category
INNER JOIN story_category ON category_id=category.id);
But I get the next error:
#1093 - You can't specify target table 'story_category' for update in FROM clause
How can I overcome this?
| Update: This answer covers the general error classification. For a more specific answer about how to best handle the OP's exact query, please see other answers to this question
In MySQL, you can't modify the same table which you use in the SELECT part.
This behaviour is documented at:
http://dev.mysql.com/doc/refman/5.6/en/update.html
Maybe you can just join the table to itself
If the logic is simple enough to re-shape the query, lose the subquery and join the table to itself, employing appropriate selection criteria. This will cause MySQL to see the table as two different things, allowing destructive changes to go ahead.
UPDATE tbl AS a
INNER JOIN tbl AS b ON ....
SET a.col = b.col
Alternatively, try nesting the subquery deeper into a from clause ...
If you absolutely need the subquery, there's a workaround, but it's
ugly for several reasons, including performance:
UPDATE tbl SET col = (
SELECT ... FROM (SELECT.... FROM) AS x);
The nested subquery in the FROM clause creates an implicit temporary
table, so it doesn't count as the same table you're updating.
... but watch out for the query optimiser
However, beware that from MySQL 5.7.6 and onward, the optimiser may optimise out the subquery, and still give you the error. Luckily, the optimizer_switch variable can be used to switch off this behaviour; although I couldn't recommend doing this as anything more than a short term fix, or for small one-off tasks.
SET optimizer_switch = 'derived_merge=off';
Thanks to Peter V. Mørch for this advice in the comments.
Example technique was from Baron Schwartz, originally published at Nabble, paraphrased and extended here.
| MySQL | 45,494 | 839 |
Is it possible to temporarily disable constraints in MySQL?
I have two Django models, each with a foreign key to the other one. Deleting instances of a model returns an error because of the foreign key constraint:
cursor.execute("DELETE FROM myapp_item WHERE n = %s", n)
transaction.commit_unless_managed() #a foreign key constraint fails here
cursor.execute("DELETE FROM myapp_style WHERE n = %s", n)
transaction.commit_unless_managed()
Is it possible to temporarily disable constraints and delete anyway?
| Try DISABLE KEYS or
SET FOREIGN_KEY_CHECKS=0;
Make sure to
SET FOREIGN_KEY_CHECKS=1;
after.
| MySQL | 15,501,673 | 837 |
How to get size of a mysql database?
Suppose the target database is called "v3".
| Run this query and you'll probably get what you're looking for:
SELECT table_schema "DB Name",
ROUND(SUM(data_length + index_length) / 1024 / 1024, 1) "DB Size in MB"
FROM information_schema.tables
GROUP BY table_schema;
This query comes from the mysql forums, where there are more comprehensive instructions available.
| MySQL | 1,733,507 | 815 |
Is it possible to make a simple query to count how many records I have in a determined period of time like a year, month, or day, having a TIMESTAMP field, like:
SELECT COUNT(id)
FROM stats
WHERE record_date.YEAR = 2009
GROUP BY record_date.YEAR
Or even:
SELECT COUNT(id)
FROM stats
GROUP BY record_date.YEAR, record_date.MONTH
To have a monthly statistic.
Thanks!
| GROUP BY YEAR(record_date), MONTH(record_date)
Check out the date and time functions in MySQL.
| MySQL | 508,791 | 800 |
Is there a collation type which is officially recommended by MySQL, for a general website where you aren't 100% sure of what will be entered? I understand that all the encodings should be the same, such as MySQL, Apache, the HTML and anything inside PHP.
In the past I have set PHP to output in "UTF-8", but which collation does this match in MySQL? I'm thinking it's one of the UTF-8 ones, but I have used utf8_unicode_ci, utf8_general_ci, and utf8_bin before, and I do not know which of these "utf8" maps to, or if that is the best to use.
| The main difference is sorting accuracy (when comparing characters in the language) and performance. The only special one is utf8_bin which is for comparing characters in binary format.
utf8_general_ci is somewhat faster than utf8_unicode_ci, but less accurate (for sorting). The specific language utf8 encoding (such as utf8_swedish_ci) contain additional language rules that make them the most accurate to sort for those languages. Most of the time I use utf8_unicode_ci (I prefer accuracy to small performance improvements), unless I have a good reason to prefer a specific language.
You can read more on specific unicode character sets on the MySQL manual - http://dev.mysql.com/doc/refman/5.0/en/charset-unicode-sets.html
| MySQL | 367,711 | 796 |
In MySQL, how do I get a list of all foreign key constraints pointing to a particular table? a particular column? This is the same thing as this Oracle question, but for MySQL.
| For a Table:
SELECT
TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME, REFERENCED_TABLE_NAME,REFERENCED_COLUMN_NAME
FROM
INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE
REFERENCED_TABLE_SCHEMA = (SELECT DATABASE()) AND
REFERENCED_TABLE_NAME = '<table>' \G
For a Column:
SELECT
TABLE_NAME,COLUMN_NAME,CONSTRAINT_NAME, REFERENCED_TABLE_NAME,REFERENCED_COLUMN_NAME
FROM
INFORMATION_SCHEMA.KEY_COLUMN_USAGE
WHERE
REFERENCED_TABLE_SCHEMA = (SELECT DATABASE()) AND
REFERENCED_TABLE_NAME = '<table>' AND
REFERENCED_COLUMN_NAME = '<column>' \G
Basically, change REFERENCED_TABLE_NAME with REFERENCED_COLUMN_NAME in the WHERE clause.
| MySQL | 201,621 | 796 |
I am trying to learn the best way to write queries. I also understand the importance of being consistent. Until now, I have randomly used single quotes, double quotes, and backticks without any real thought.
Example:
$query = 'INSERT INTO table (id, col1, col2) VALUES (NULL, val1, val2)';
Also, in the above example, consider that table, col1, val1, etc. may be variables.
What is the standard for this? What do you do?
| Backticks are to be used for table and column identifiers, but are only necessary when the identifier is a MySQL reserved keyword, or when the identifier contains whitespace characters or characters beyond a limited set (see below) It is often recommended to avoid using reserved keywords as column or table identifiers when possible, avoiding the quoting issue.
Single quotes should be used for string values like in the VALUES() list. Double quotes are supported by MySQL for string values as well, but single quotes are more widely accepted by other RDBMS, so it is a good habit to use single quotes instead of double.
MySQL also expects DATE and DATETIME literal values to be single-quoted as strings like '2001-01-01 00:00:00'. Consult the Date and Time Literals documentation for more details, in particular alternatives to using the hyphen - as a segment delimiter in date strings.
So using your example, I would double-quote the PHP string and use single quotes on the values 'val1', 'val2'. NULL is a MySQL keyword, and a special (non)-value, and is therefore unquoted.
None of these table or column identifiers are reserved words or make use of characters requiring quoting, but I've quoted them anyway with backticks (more on this later...).
Functions native to the RDBMS (for example, NOW() in MySQL) should not be quoted, although their arguments are subject to the same string or identifier quoting rules already mentioned.
Backtick (`)
table & column ───────┬─────┬──┬──┬──┬────┬──┬────┬──┬────┬──┬───────┐
↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓ ↓
$query = "INSERT INTO `table` (`id`, `col1`, `col2`, `date`, `updated`)
VALUES (NULL, 'val1', 'val2', '2001-01-01', NOW())";
↑↑↑↑ ↑ ↑ ↑ ↑ ↑ ↑ ↑↑↑↑↑
Unquoted keyword ─────┴┴┴┘ │ │ │ │ │ │ │││││
Single-quoted (') strings ───────────┴────┴──┴────┘ │ │ │││││
Single-quoted (') DATE ───────────────────────────┴──────────┘ │││││
Unquoted function ─────────────────────────────────────────┴┴┴┴┘
Variable interpolation
The quoting patterns for variables do not change, although if you intend to interpolate the variables directly in a string, it must be double-quoted in PHP. Just make sure that you have properly escaped the variables for use in SQL. (It is recommended to use an API supporting prepared statements instead, as protection against SQL injection).
// Same thing with some variable replacements
// Here, a variable table name $table is backtick-quoted, and variables
// in the VALUES list are single-quoted
$query = "INSERT INTO `$table` (`id`, `col1`, `col2`, `date`) VALUES (NULL, '$val1', '$val2', '$date')";
Prepared statements
When working with prepared statements, consult the documentation to determine whether or not the statement's placeholders must be quoted. The most popular APIs available in PHP, PDO and MySQLi, expect unquoted placeholders, as do most prepared statement APIs in other languages:
// PDO example with named parameters, unquoted
$query = "INSERT INTO `table` (`id`, `col1`, `col2`, `date`) VALUES (:id, :col1, :col2, :date)";
// MySQLi example with ? parameters, unquoted
$query = "INSERT INTO `table` (`id`, `col1`, `col2`, `date`) VALUES (?, ?, ?, ?)";
Characters requring backtick quoting in identifiers:
According to MySQL documentation, you do not need to quote (backtick) identifiers using the following character set:
ASCII: [0-9,a-z,A-Z$_] (basic Latin letters, digits 0-9, dollar, underscore)
You can use characters beyond that set as table or column identifiers, including whitespace for example, but then you must quote (backtick) them.
Also, although numbers are valid characters for identifiers, identifiers cannot consist solely of numbers. If they do they must be wrapped in backticks.
| MySQL | 11,321,491 | 794 |
Is there an SQL injection possibility even when using mysql_real_escape_string() function?
Consider this sample situation. SQL is constructed in PHP like this:
$login = mysql_real_escape_string(GetFromPost('login'));
$password = mysql_real_escape_string(GetFromPost('password'));
$sql = "SELECT * FROM table WHERE login='$login' AND password='$password'";
I have heard numerous people say to me that code like that is still dangerous and possible to hack even with mysql_real_escape_string() function used. But I cannot think of any possible exploit?
Classic injections like this:
aaa' OR 1=1 --
do not work.
Do you know of any possible injection that would get through the PHP code above?
| The short answer is yes, yes there is a way to get around mysql_real_escape_string().
#For Very OBSCURE EDGE CASES!!!
The long answer isn't so easy. It's based off an attack demonstrated here.
The Attack
So, let's start off by showing the attack...
mysql_query('SET NAMES gbk');
$var = mysql_real_escape_string("\xbf\x27 OR 1=1 /*");
mysql_query("SELECT * FROM test WHERE name = '$var' LIMIT 1");
In certain circumstances, that will return more than 1 row. Let's dissect what's going on here:
Selecting a Character Set
mysql_query('SET NAMES gbk');
For this attack to work, we need the encoding that the server's expecting on the connection both to encode ' as in ASCII i.e. 0x27 and to have some character whose final byte is an ASCII \ i.e. 0x5c. As it turns out, there are 5 such encodings supported in MySQL 5.6 by default: big5, cp932, gb2312, gbk and sjis. We'll select gbk here.
Now, it's very important to note the use of SET NAMES here. This sets the character set ON THE SERVER. If we used the call to the C API function mysql_set_charset(), we'd be fine (on MySQL releases since 2006). But more on why in a minute...
The Payload
The payload we're going to use for this injection starts with the byte sequence 0xbf27. In gbk, that's an invalid multibyte character; in latin1, it's the string ¿'. Note that in latin1 and gbk, 0x27 on its own is a literal ' character.
We have chosen this payload because, if we called addslashes() on it, we'd insert an ASCII \ i.e. 0x5c, before the ' character. So we'd wind up with 0xbf5c27, which in gbk is a two character sequence: 0xbf5c followed by 0x27. Or in other words, a valid character followed by an unescaped '. But we're not using addslashes(). So on to the next step...
mysql_real_escape_string()
The C API call to mysql_real_escape_string() differs from addslashes() in that it knows the connection character set. So it can perform the escaping properly for the character set that the server is expecting. However, up to this point, the client thinks that we're still using latin1 for the connection, because we never told it otherwise. We did tell the server we're using gbk, but the client still thinks it's latin1.
Therefore the call to mysql_real_escape_string() inserts the backslash, and we have a free hanging ' character in our "escaped" content! In fact, if we were to look at $var in the gbk character set, we'd see:
縗' OR 1=1 /*
Which is exactly what the attack requires.
The Query
This part is just a formality, but here's the rendered query:
SELECT * FROM test WHERE name = '縗' OR 1=1 /*' LIMIT 1
Congratulations, you just successfully attacked a program using mysql_real_escape_string()...
The Bad
It gets worse. PDO defaults to emulating prepared statements with MySQL. That means that on the client side, it basically does a sprintf through mysql_real_escape_string() (in the C library), which means the following will result in a successful injection:
$pdo->query('SET NAMES gbk');
$stmt = $pdo->prepare('SELECT * FROM test WHERE name = ? LIMIT 1');
$stmt->execute(array("\xbf\x27 OR 1=1 /*"));
Now, it's worth noting that you can prevent this by disabling emulated prepared statements:
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
This will usually result in a true prepared statement (i.e. the data being sent over in a separate packet from the query). However, be aware that PDO will silently fallback to emulating statements that MySQL can't prepare natively: those that it can are listed in the manual, but beware to select the appropriate server version).
The Ugly
I said at the very beginning that we could have prevented all of this if we had used mysql_set_charset('gbk') instead of SET NAMES gbk. And that's true provided you are using a MySQL release since 2006.
If you're using an earlier MySQL release, then a bug in mysql_real_escape_string() meant that invalid multibyte characters such as those in our payload were treated as single bytes for escaping purposes even if the client had been correctly informed of the connection encoding and so this attack would still succeed. The bug was fixed in MySQL 4.1.20, 5.0.22 and 5.1.11.
But the worst part is that PDO didn't expose the C API for mysql_set_charset() until 5.3.6, so in prior versions it cannot prevent this attack for every possible command!
It's now exposed as a DSN parameter.
The Saving Grace
As we said at the outset, for this attack to work the database connection must be encoded using a vulnerable character set. utf8mb4 is not vulnerable and yet can support every Unicode character: so you could elect to use that instead—but it has only been available since MySQL 5.5.3. An alternative is utf8, which is also not vulnerable and can support the whole of the Unicode Basic Multilingual Plane.
Alternatively, you can enable the NO_BACKSLASH_ESCAPES SQL mode, which (amongst other things) alters the operation of mysql_real_escape_string(). With this mode enabled, 0x27 will be replaced with 0x2727 rather than 0x5c27 and thus the escaping process cannot create valid characters in any of the vulnerable encodings where they did not exist previously (i.e. 0xbf27 is still 0xbf27 etc.)—so the server will still reject the string as invalid. However, see @eggyal's answer for a different vulnerability that can arise from using this SQL mode.
Safe Examples
The following examples are safe:
mysql_query('SET NAMES utf8');
$var = mysql_real_escape_string("\xbf\x27 OR 1=1 /*");
mysql_query("SELECT * FROM test WHERE name = '$var' LIMIT 1");
Because the server's expecting utf8...
mysql_set_charset('gbk');
$var = mysql_real_escape_string("\xbf\x27 OR 1=1 /*");
mysql_query("SELECT * FROM test WHERE name = '$var' LIMIT 1");
Because we've properly set the character set so the client and the server match.
$pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
$pdo->query('SET NAMES gbk');
$stmt = $pdo->prepare('SELECT * FROM test WHERE name = ? LIMIT 1');
$stmt->execute(array("\xbf\x27 OR 1=1 /*"));
Because we've turned off emulated prepared statements.
$pdo = new PDO('mysql:host=localhost;dbname=testdb;charset=gbk', $user, $password);
$stmt = $pdo->prepare('SELECT * FROM test WHERE name = ? LIMIT 1');
$stmt->execute(array("\xbf\x27 OR 1=1 /*"));
Because we've set the character set properly.
$mysqli->query('SET NAMES gbk');
$stmt = $mysqli->prepare('SELECT * FROM test WHERE name = ? LIMIT 1');
$param = "\xbf\x27 OR 1=1 /*";
$stmt->bind_param('s', $param);
$stmt->execute();
Because MySQLi does true prepared statements all the time.
Wrapping Up
If you:
Use Modern Versions of MySQL (late 5.1, all 5.5, 5.6, etc) AND mysql_set_charset() / $mysqli->set_charset() / PDO's DSN charset parameter (in PHP ≥ 5.3.6)
OR
Don't use a vulnerable character set for connection encoding (you only use utf8 / latin1 / ascii / etc)
You're 100% safe.
Otherwise, you're vulnerable even though you're using mysql_real_escape_string()...
| MySQL | 5,741,187 | 788 |
I can't make a simple connection to the server for some reason. I install the newest MySQL Community 8.0 database along with Node.JS with default settings.
This is my node.js code
var mysql = require('mysql');
var con = mysql.createConnection({
host: "localhost",
user: "root",
password: "password",
insecureAuth : true
});
con.connect(function(err) {
if (err) throw err;
console.log("Connected!");
});
Below is the error found in Command Prompt:
C:\Users\mysql-test>node app.js
C:\Users\mysql-test\node_modules\mysql\lib\protocol\Parse
r.js:80
throw err; // Rethrow non-MySQL errors
^
Error: ER_NOT_SUPPORTED_AUTH_MODE: Client does not support authentication protocol requested by server; consider upgrading MySQL client
at Handshake.Sequence._packetToError (C:\Users\mysql-
test\node_modules\mysql\lib\protocol\sequences\Sequence.js:52:14)
at Handshake.ErrorPacket (C:\Users\mysql-test\node_mo
dules\mysql\lib\protocol\sequences\Handshake.js:130:18)
at Protocol._parsePacket (C:\Users\mysql-test\node_mo
dules\mysql\lib\protocol\Protocol.js:279:23)
at Parser.write (C:\Users\mysql-test\node_modules\mys
ql\lib\protocol\Parser.js:76:12)
at Protocol.write (C:\Users\mysql-test\node_modules\m
ysql\lib\protocol\Protocol.js:39:16)
at Socket.<anonymous> (C:\Users\mysql-test\node_modul
es\mysql\lib\Connection.js:103:28)
at Socket.emit (events.js:159:13)
at addChunk (_stream_readable.js:265:12)
at readableAddChunk (_stream_readable.js:252:11)
at Socket.Readable.push (_stream_readable.js:209:10)
--------------------
at Protocol._enqueue (C:\Users\mysql-test\node_module
s\mysql\lib\protocol\Protocol.js:145:48)
at Protocol.handshake (C:\Users\mysql-test\node_modul
es\mysql\lib\protocol\Protocol.js:52:23)
at Connection.connect (C:\Users\mysql-test\node_modul
es\mysql\lib\Connection.js:130:18)
at Object.<anonymous> (C:\Users\mysql-test\server.js:
11:5)
at Module._compile (module.js:660:30)
at Object.Module._extensions..js (module.js:671:10)
at Module.load (module.js:573:32)
at tryModuleLoad (module.js:513:12)
at Function.Module._load (module.js:505:3)
at Function.Module.runMain (module.js:701:10)
I've read up on some things such as:
https://dev.mysql.com/doc/refman/5.5/en/old-client.html
https://github.com/mysqljs/mysql/issues/1507
But I am still not sure how to fix my problem.
| Execute the following query in MYSQL Workbench
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'password';
Where root as your user
localhost as your URL
and password as your password
Then run this query to refresh privileges:
flush privileges;
Try connecting using node after you do so.
If that doesn't work, try it without @'localhost' part.
| MySQL | 50,093,144 | 781 |
SELECT id, amount FROM report
I need amount to be amount if report.type='P' and -amount if report.type='N'. How do I add this to the above query?
| SELECT id,
IF(type = 'P', amount, amount * -1) as amount
FROM report
See https://dev.mysql.com/doc/refman/8.0/en/flow-control-functions.html.
Additionally, you could handle when the condition is null. In the case of a null amount:
SELECT id,
IF(type = 'P', IFNULL(amount,0), IFNULL(amount,0) * -1) as amount
FROM report
The part IFNULL(amount,0) means when amount is not null return amount else return 0.
| MySQL | 5,951,157 | 765 |
I want to pull out duplicate records in a MySQL Database. This can be done with:
SELECT address, count(id) as cnt FROM list
GROUP BY address HAVING cnt > 1
Which results in:
100 MAIN ST 2
I would like to pull it so that it shows each row that is a duplicate. Something like:
JIM JONES 100 MAIN ST
JOHN SMITH 100 MAIN ST
Any thoughts on how this can be done? I'm trying to avoid doing the first one then looking up the duplicates with a second query in the code.
| The key is to rewrite this query so that it can be used as a subquery.
SELECT firstname,
lastname,
list.address
FROM list
INNER JOIN (SELECT address
FROM list
GROUP BY address
HAVING COUNT(id) > 1) dup
ON list.address = dup.address;
| MySQL | 854,128 | 743 |
I want to change the data type of multiple columns from float to int. What is the simplest way to do this?
There is no data to worry about, yet.
| http://dev.mysql.com/doc/refman/5.1/en/alter-table.html
ALTER TABLE tablename MODIFY columnname INTEGER;
This will change the datatype of given column
Depending on how many columns you wish to modify it might be best to generate a script, or use some kind of mysql client GUI
| MySQL | 1,356,866 | 690 |
I need to check (from the same table) if there is an association between two events based on date-time.
One set of data will contain the ending date-time of certain events and the other set of data will contain the starting date-time for other events.
If the first event completes before the second event then I would like to link them up.
What I have so far is:
SELECT name as name_A, date-time as end_DTS, id as id_A
FROM tableA WHERE criteria = 1
SELECT name as name_B, date-time as start_DTS, id as id_B
FROM tableA WHERE criteria = 2
Then I join them:
SELECT name_A, name_B, id_A, id_B,
if(start_DTS > end_DTS,'VALID','') as validation_check
FROM tableA
LEFT JOIN tableB ON name_A = name_B
Can I then, based on my validation_check field, run a UPDATE query with the SELECT nested?
| You can actually do this one of two ways:
MySQL update join syntax:
UPDATE tableA a
INNER JOIN tableB b ON a.name_a = b.name_b
SET validation_check = if(start_dts > end_dts, 'VALID', '')
-- where clause can go here
ANSI SQL syntax:
UPDATE tableA SET validation_check =
(SELECT if(start_DTS > end_DTS, 'VALID', '') AS validation_check
FROM tableA
INNER JOIN tableB ON name_A = name_B
WHERE id_A = tableA.id_A)
Pick whichever one seems most natural to you.
| MySQL | 1,262,786 | 690 |
Is there a way to restrict certain tables from the mysqldump command?
For example, I'd use the following syntax to dump only table1 and table2:
mysqldump -u username -p database table1 table2 > database.sql
But is there a similar way to dump all the tables except table1 and table2? I haven't found anything in the mysqldump documentation, so is brute-force (specifying all the table names) the only way to go?
| You can use the --ignore-table option. So you could do
mysqldump -u USERNAME -pPASSWORD DATABASE --ignore-table=DATABASE.table1 > database.sql
There is no whitespace after -p (this is not a typo).
To ignore multiple tables, use this option multiple times, this is documented to work since at least version 5.0.
If you want an alternative way to ignore multiple tables you can use a script like this:
#!/bin/bash
PASSWORD=XXXXXX
HOST=XXXXXX
USER=XXXXXX
DATABASE=databasename
DB_FILE=dump.sql
EXCLUDED_TABLES=(
table1
table2
table3
table4
tableN
)
IGNORED_TABLES_STRING=''
for TABLE in "${EXCLUDED_TABLES[@]}"
do :
IGNORED_TABLES_STRING+=" --ignore-table=${DATABASE}.${TABLE}"
done
echo "Dump structure"
mysqldump --host=${HOST} --user=${USER} --password=${PASSWORD} --single-transaction --no-data --routines ${DATABASE} > ${DB_FILE}
echo "Dump content"
mysqldump --host=${HOST} --user=${USER} --password=${PASSWORD} ${DATABASE} --no-create-info --skip-triggers ${IGNORED_TABLES_STRING} >> ${DB_FILE}
| MySQL | 425,158 | 682 |
I am connecting MySQL - 8.0 with MySQL Workbench and getting the below error:
Authentication plugin 'caching_sha2_password' cannot be loaded:
dlopen(/usr/local/mysql/lib/plugin/caching_sha2_password.so, 2): image
not found
I have tried with other client tool as well.
Any solution for this?
| You can change the encryption of the password like this.
ALTER USER 'yourusername'@'localhost' IDENTIFIED WITH mysql_native_password BY 'youpassword';
| MySQL | 49,194,719 | 681 |
I'm using a MySql database with a Java program, now I want to give the program to somebody else.
How to export the MySQL database structure without the data in it, just the structure?
| You can do with the --no-data option with mysqldump command
mysqldump -h yourhostnameorIP -u root -p --no-data dbname > schema.sql
| MySQL | 6,175,473 | 676 |
I was given a MySQL database file that I need to restore as a database on my Windows Server 2008 machine.
I tried using MySQL Administrator, but I got the following error:
The selected file was generated by
mysqldump and cannot be restored by
this application.
How do I get this working?
| If the database you want to restore doesn't already exist, you need to create it first.
On the command-line, if you're in the same directory that contains the dumped file, use these commands (with appropriate substitutions):
C:\> mysql -u root -p
mysql> create database mydb;
mysql> use mydb;
mysql> source db_backup.dump;
| MySQL | 105,776 | 675 |
When I executed the following command:
ALTER TABLE `mytable` ADD UNIQUE (
`column1` ,
`column2`
);
I got this error message:
#1071 - Specified key was too long; max key length is 767 bytes
Information about column1 and column2:
column1 varchar(20) utf8_general_ci
column2 varchar(500) utf8_general_ci
I think varchar(20) only requires 21 bytes while varchar(500) only requires 501 bytes. So the total bytes are 522, less than 767. So why did I get the error message?
#1071 - Specified key was too long; max key length is 767 bytes
| 767 bytes in MySQL version 5.6 (and prior versions), is the stated prefix limitation for InnoDB tables. It's 1,000 bytes long for MyISAM tables. This limit has been increased to 3072 bytes In MySQL version 5.7 (and upwards).
You also have to be aware that if you set an index on a big char or varchar field which is utf8mb4 encoded, you have to divide the max index prefix length of 767 bytes (or 3072 bytes) by 4 resulting in 191. This is because the maximum length of a utf8mb4 character is four bytes. For a utf8 character it would be three bytes resulting in max index prefix length of 255 (or minus null-terminator, 254 characters).
One option you have is to just place lower limit on your VARCHAR fields.
Another option (according to the response to this issue) is to get the subset of the column rather than the entire amount, i.e.:
ALTER TABLE `mytable` ADD UNIQUE ( column1(15), column2(200) );
Tweak as you need to get the key to apply, but I wonder if it would be worth it to review your data model regarding this entity to see if there's improvements possible, which would allow you to implement the intended business rules without hitting the MySQL limitation.
| MySQL | 1,814,532 | 672 |
I have a column containing urls (id, url):
http://www.example.com/articles/updates/43
http://www.example.com/articles/updates/866
http://www.example.com/articles/updates/323
http://www.example.com/articles/updates/seo-url
http://www.example.com/articles/updates/4?something=test
I'd like to change the word "updates" to "news". Is it possible to do this with a script?
| UPDATE your_table
SET your_field = REPLACE(your_field, 'articles/updates/', 'articles/news/')
WHERE your_field LIKE '%articles/updates/%'
Now rows that were like
http://www.example.com/articles/updates/43
will be
http://www.example.com/articles/news/43
http://www.electrictoolbox.com/mysql-find-replace-text/
| MySQL | 5,956,993 | 651 |
What is the size of column of int(11) in mysql in bytes?
And Maximum value that can be stored in this columns?
| An INT will always be 4 bytes no matter what length is specified.
TINYINT = 1 byte (8 bit)
SMALLINT = 2 bytes (16 bit)
MEDIUMINT = 3 bytes (24 bit)
INT = 4 bytes (32 bit)
BIGINT = 8 bytes (64 bit).
The length just specifies how many characters to pad when selecting data with the mysql command line client. 12345 stored as int(3) will still show as 12345, but if it was stored as int(10) it would still display as 12345, but you would have the option to pad the first five digits. For example, if you added ZEROFILL it would display as 0000012345.
... and the maximum value will be 2147483647 (Signed) or 4294967295 (Unsigned)
| MySQL | 5,634,104 | 640 |
What command returns the current version of a MySQL database?
| Try this function -
SELECT VERSION();
-> '5.7.22-standard'
VERSION()
Or for more details use :
SHOW VARIABLES LIKE "%version%";
+-------------------------+------------------------------------------+
| Variable_name | Value |
+-------------------------+------------------------------------------+
| protocol_version | 10 |
| version | 5.0.27-standard |
| version_comment | MySQL Community Edition - Standard (GPL) |
| version_compile_machine | i686 |
| version_compile_os | pc-linux-gnu |
+-------------------------+------------------------------------------+
5 rows in set (0.04 sec)
MySQL 5.0 Reference Manual (pdf) - Determining Your Current MySQL Version - page 42
| MySQL | 8,987,679 | 628 |
How can I trace MySQL queries on my Linux server as they happen?
For example I'd love to set up some sort of listener, then request a web page and view all of the queries the engine executed, or just view all of the queries being run on a production server. How can I do this?
| You can log every query to a log file really easily:
mysql> SHOW VARIABLES LIKE "general_log%";
+------------------+----------------------------+
| Variable_name | Value |
+------------------+----------------------------+
| general_log | OFF |
| general_log_file | /var/run/mysqld/mysqld.log |
+------------------+----------------------------+
mysql> SET GLOBAL general_log = 'ON';
Do your queries (on any db). Grep or otherwise examine /var/run/mysqld/mysqld.log
Then don't forget to
mysql> SET GLOBAL general_log = 'OFF';
or the performance will plummet and your disk will fill!
| MySQL | 568,564 | 621 |
I have a table with ~500k rows; varchar(255) UTF8 column filename contains a file name;
I'm trying to strip out various strange characters out of the filename - thought I'd use a character class: [^a-zA-Z0-9()_ .\-]
Now, is there a function in MySQL that lets you replace through a regular expression? I'm looking for a similar functionality to REPLACE() function - simplified example follows:
SELECT REPLACE('stackowerflow', 'ower', 'over');
Output: "stackoverflow"
/* does something like this exist? */
SELECT X_REG_REPLACE('Stackoverflow','/[A-Zf]/','-');
Output: "-tackover-low"
I know about REGEXP/RLIKE, but those only check if there is a match, not what the match is.
(I could do a "SELECT pkey_id,filename FROM foo WHERE filename RLIKE '[^a-zA-Z0-9()_ .\-]'" from a PHP script, do a preg_replace and then "UPDATE foo ... WHERE pkey_id=...", but that looks like a last-resort slow & ugly hack)
| If you are using MariaDB or MySQL 8.0, they have a function
REGEXP_REPLACE(col, regexp, replace)
See MariaDB docs and PCRE Regular expression enhancements
Note that you can use regexp grouping as well (I found that very useful):
SELECT REGEXP_REPLACE("stackoverflow", "(stack)(over)(flow)", '\\2 - \\1 - \\3')
returns
over - stack - flow
| MySQL | 986,826 | 616 |
I'm setting up a new server and keep running into this problem.
When I try to log into the MySQL database with the root user, I get the error:
ERROR 1698 (28000): Access denied for user 'root'@'localhost'
It doesn't matter if I connect through the terminal (SSH), through phpMyAdmin or a MySQL client, e.g., Navicat. They all fail.
I looked in the mysql.user table and get the following:
+------------------+-------------------+
| user | host |
+------------------+-------------------+
| root | % |
| root | 127.0.0.1 |
| amavisd | localhost |
| debian-sys-maint | localhost |
| iredadmin | localhost |
| iredapd | localhost |
| mysql.sys | localhost |
| phpmyadmin | localhost |
| root | localhost |
| roundcube | localhost |
| vmail | localhost |
| vmailadmin | localhost |
| amavisd | test4.folkmann.it |
| iredadmin | test4.folkmann.it |
| iredapd | test4.folkmann.it |
| roundcube | test4.folkmann.it |
| vmail | test4.folkmann.it |
| vmailadmin | test4.folkmann.it |
+------------------+-------------------+
As you can see, user root should have access.
The Server is quite simple, as I have tried to troubleshoot this for a while now.
It's running Ubuntu 16.04.1 LTS (Xenial Xerus) with Apache, MySQL and PHP, so that it can host websites, and iRedMail 0.9.5-1, so that it can host mail.
Log into the MySQL database works fine before I installed iRedMail. I also tried just installing iRedMail, but then root also doesn't work.
How can I fix my MySQL login problem or how can I install iRedMail over an existing MySQL install? And yes, I tried the Installation Tips and I can't find those variables in the configuration files.
| On some systems, like Ubuntu, MySQL is using the Unix auth_socket plugin by default.
Basically it means that: db_users using it, will be "authenticated" by the system user credentials. You can see if your root user is set up like this by doing the following:
sudo mysql -u root # I had to use "sudo" since it was a new installation
mysql> USE mysql;
mysql> SELECT User, Host, plugin FROM mysql.user;
+------------------+-----------------------+
| User | plugin |
+------------------+-----------------------+
| root | auth_socket |
| mysql.sys | mysql_native_password |
| debian-sys-maint | mysql_native_password |
+------------------+-----------------------+
As you can see in the query, the root user is using the auth_socket plugin.
There are two ways to solve this:
You can set the root user to use the mysql_native_password plugin
You can create a new db_user with you system_user (recommended)
Option 1:
sudo mysql -u root # I had to use "sudo" since it was a new installation
mysql> USE mysql;
mysql> UPDATE user SET plugin='mysql_native_password' WHERE User='root';
mysql> FLUSH PRIVILEGES;
mysql> exit;
sudo service mysql restart
Option 2: (replace YOUR_SYSTEM_USER with the username you have)
sudo mysql -u root # I had to use "sudo" since it was a new installation
mysql> USE mysql;
mysql> CREATE USER 'YOUR_SYSTEM_USER'@'localhost' IDENTIFIED BY 'YOUR_PASSWD';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'YOUR_SYSTEM_USER'@'localhost';
mysql> UPDATE user SET plugin='auth_socket' WHERE User='YOUR_SYSTEM_USER';
mysql> FLUSH PRIVILEGES;
mysql> exit;
sudo service mysql restart
Remember that if you use option #2 you'll have to connect to MySQL as your system username (mysql -u YOUR_SYSTEM_USER).
Note: On some systems (e.g., Debian 9 (Stretch)) the 'auth_socket' plugin is called 'unix_socket', so the corresponding SQL command should be: UPDATE user SET plugin='unix_socket' WHERE User='YOUR_SYSTEM_USER';
From andy's comment it seems that MySQL 8.x.x updated/replaced the auth_socket for caching_sha2_password. I don't have a system setup with MySQL 8.x.x to test this. However, the steps above should help you to understand the issue. Here's the reply:
One change as of MySQL 8.0.4 is that the new default authentication plugin is 'caching_sha2_password'. The new 'YOUR_SYSTEM_USER' will have this authentication plugin and you can log in from the Bash shell now with "mysql -u YOUR_SYSTEM_USER -p" and provide the password for this user on the prompt. There isn’t any need for the "UPDATE user SET plugin" step.
For the 8.0.4 default authentication plugin update, see MySQL 8.0.4: New Default Authentication Plugin: caching_sha2_password.
| MySQL | 39,281,594 | 614 |
Here is the script to create my tables:
CREATE TABLE clients (
client_i INT(11),
PRIMARY KEY (client_id)
);
CREATE TABLE projects (
project_id INT(11) UNSIGNED,
client_id INT(11) UNSIGNED,
PRIMARY KEY (project_id)
);
CREATE TABLE posts (
post_id INT(11) UNSIGNED,
project_id INT(11) UNSIGNED,
PRIMARY KEY (post_id)
);
In my PHP code, when deleting a client, I want to delete all projects posts:
DELETE
FROM posts
INNER JOIN projects ON projects.project_id = posts.project_id
WHERE projects.client_id = :client_id;
The posts table does not have a foreign key client_id, only project_id. I want to delete the posts in projects that have the passed client_id.
This is not working right now because no posts are deleted.
| You just need to specify that you want to delete the entries from the posts table:
DELETE posts
FROM posts
INNER JOIN projects ON projects.project_id = posts.project_id
WHERE projects.client_id = :client_id
EDIT: For more information you can see this alternative answer
| MySQL | 652,770 | 605 |
How do I see if my database has any indexes on it?
How about for a specific table?
| To see the index for a specific table use SHOW INDEX:
SHOW INDEX FROM yourtable;
To see indexes for all tables within a specific schema you can use the STATISTICS table from INFORMATION_SCHEMA:
SELECT DISTINCT
TABLE_NAME,
INDEX_NAME
FROM INFORMATION_SCHEMA.STATISTICS
WHERE TABLE_SCHEMA = 'your_schema';
Removing the where clause will show you all indexes in all schemas.
| MySQL | 5,213,339 | 604 |
How to declare a variable in mysql, so that my second query can use it?
I would like to write something like:
SET start = 1;
SET finish = 10;
SELECT * FROM places WHERE place BETWEEN start AND finish;
| There are mainly three types of variables in MySQL:
User-defined variables (prefixed with @):
You can access any user-defined variable without declaring it or
initializing it. If you refer to a variable that has not been
initialized, it has a value of NULL and a type of string.
SELECT @var_any_var_name
You can initialize a variable using SET or SELECT statement:
SET @start = 1, @finish = 10;
or
SELECT @start := 1, @finish := 10;
SELECT * FROM places WHERE place BETWEEN @start AND @finish;
User variables can be assigned a value from a limited set of data
types: integer, decimal, floating-point, binary or nonbinary string,
or NULL value.
User-defined variables are session-specific. That is, a user
variable defined by one client cannot be seen or used by other
clients.
They can be used in SELECT queries using Advanced MySQL user variable techniques.
Local Variables (no prefix) :
Local variables needs to be declared using DECLARE before
accessing it.
They can be used as local variables and the input parameters
inside a stored procedure:
DELIMITER //
CREATE PROCEDURE sp_test(var1 INT)
BEGIN
DECLARE start INT unsigned DEFAULT 1;
DECLARE finish INT unsigned DEFAULT 10;
SELECT var1, start, finish;
SELECT * FROM places WHERE place BETWEEN start AND finish;
END; //
DELIMITER ;
CALL sp_test(5);
If the DEFAULT clause is missing, the initial value is NULL.
The scope of a local variable is the BEGIN ... END block within
which it is declared.
Server System Variables (prefixed with @@):
The MySQL server maintains many system variables configured to a default value.
They can be of type GLOBAL, SESSION or BOTH.
Global variables affect the overall operation of the server whereas session variables affect its operation for individual client connections.
To see the current values used by a running server, use the SHOW VARIABLES statement or SELECT @@var_name.
SHOW VARIABLES LIKE '%wait_timeout%';
SELECT @@sort_buffer_size;
They can be set at server startup using options on the command line or in an option file.
Most of them can be changed dynamically while the server is running using SET GLOBAL or SET SESSION:
-- Syntax to Set value to a Global variable:
SET GLOBAL sort_buffer_size=1000000;
SET @@global.sort_buffer_size=1000000;
-- Syntax to Set value to a Session variable:
SET sort_buffer_size=1000000;
SET SESSION sort_buffer_size=1000000;
SET @@sort_buffer_size=1000000;
SET @@local.sort_buffer_size=10000;
| MySQL | 11,754,781 | 592 |
I am using MySQL in localhost as a "query tool" for performing statistics in R, that is, everytime I run a R script, I create a new database (A), create a new table (B), import the data into B, submit a query to get what I need, and then I drop B and drop A.
It's working fine for me, but I realize that the ibdata file size is increasing rapidly, I stored nothing in MySQL, but the ibdata1 file already exceeded 100 MB.
I am using more or less default MySQL setting for the setup, is there a way for I can automatically shrink/purge the ibdata1 file after a fixed period of time?
| That ibdata1 isn't shrinking is a particularly annoying feature of MySQL. The ibdata1 file can't actually be shrunk unless you delete all databases, remove the files and reload a dump.
But you can configure MySQL so that each table, including its indexes, is stored as a separate file. In that way ibdata1 will not grow as large. According to Bill Karwin's comment this is enabled by default as of version 5.6.6 of MySQL.
It was a while ago I did this. However, to setup your server to use separate files for each table you need to change my.cnf in order to enable this:
[mysqld]
innodb_file_per_table=1
https://dev.mysql.com/doc/refman/5.6/en/innodb-file-per-table-tablespaces.html
As you want to reclaim the space from ibdata1 you actually have to delete the file:
Do a mysqldump of all databases, procedures, triggers etc except the mysql and performance_schema databases.
Drop all databases except the above 2 databases(mysql and performance_schema)
Stop mysql
Delete ibdata1 and ib_log files
Start mysql
Restore from dump
When you start MySQL in step 5 the ibdata1 and ib_log files will be recreated.
Now you're fit to go. When you create a new database for analysis, the tables will be located in separate ibd* files, not in ibdata1. As you usually drop the database soon after, the ibd* files will be deleted.
http://dev.mysql.com/doc/refman/5.1/en/drop-database.html
You have probably seen this:
http://bugs.mysql.com/bug.php?id=1341
By using the command ALTER TABLE <tablename> ENGINE=innodb or OPTIMIZE TABLE <tablename> one can extract data and index pages from ibdata1 to separate files. However, ibdata1 will not shrink unless you do the steps above.
Regarding the information_schema, that is not necessary nor possible to drop. It is in fact just a bunch of read-only views, not tables. And there are no files associated with the them, not even a database directory. The informations_schema is using the memory db-engine and is dropped and regenerated upon stop/restart of mysqld. See https://dev.mysql.com/doc/refman/5.7/en/information-schema.html.
| MySQL | 3,456,159 | 581 |
Is there a nice easy way to drop all tables from a MySQL database, ignoring any foreign key constraints that may be in there?
| I found the generated set of drop statements useful, and recommend these tweaks:
Limit the generated drops to your database like this:
SELECT concat('DROP TABLE IF EXISTS `', table_name, '`;')
FROM information_schema.tables
WHERE table_schema = 'MyDatabaseName';
Note 1: This does not execute the DROP statements, it just gives you a list of them. You will need to cut and paste the output into your SQL engine to execute them.
Note 2: If you have VIEWs, you'll have to correct each DROP TABLE `VIEW_NAME` statement to DROP VIEW `VIEW_NAME` manually.
Note, per http://dev.mysql.com/doc/refman/5.5/en/drop-table.html, dropping with cascade is pointless / misleading:
"RESTRICT and CASCADE are permitted to make porting easier. In MySQL 5.5, they do nothing."
Therefore, in order for the drop statements to work if you need:
SET FOREIGN_KEY_CHECKS = 0
This will disable referential integrity checks - so when you are done performing the drops you need, you will want to reset key checking with
SET FOREIGN_KEY_CHECKS = 1
The final execution should look like:
SET FOREIGN_KEY_CHECKS = 0;
-- Your semicolon separated list of DROP statements here
SET FOREIGN_KEY_CHECKS = 1;
NB: to use output of SELECT easier, mysql -B option can help.
| MySQL | 3,476,765 | 573 |
Is it possible to create a temporary (session only) table from a select statement without using a create table statement and specifying each column type? I know derived tables are capable of this, but those are super-temporary (statement-only) and I want to re-use.
It would save time if I did not have to write up a create table command and keep the column list and type list matched up.
| CREATE TEMPORARY TABLE IF NOT EXISTS table2 AS (SELECT * FROM table1)
From the manual found at http://dev.mysql.com/doc/refman/5.7/en/create-table.html
You can use the TEMPORARY keyword when creating a table. A TEMPORARY table is visible only to the current session, and is dropped automatically when the session is closed. This means that two different sessions can use the same temporary table name without conflicting with each other or with an existing non-TEMPORARY table of the same name. (The existing table is hidden until the temporary table is dropped.) To create temporary tables, you must have the CREATE TEMPORARY TABLES privilege.
| MySQL | 5,859,391 | 571 |
By default, mysqldump takes the backup of an entire database. I need to backup a single table in MySQL. Is it possible? How do I restore it?
| Dump and restore a single table from .sql
Dump
mysqldump db_name table_name > table_name.sql
Dumping from a remote database
mysqldump -u <db_username> -h <db_host> -p db_name table_name > table_name.sql
For further reference:
http://www.abbeyworkshop.com/howto/lamp/MySQL_Export_Backup/index.html
Restore
mysql -u <user_name> -p db_name
mysql> source <full_path>/table_name.sql
or in one line
mysql -u username -p db_name < /path/to/table_name.sql
Dump and restore a single table from a compressed (.sql.gz) format
Credit: John McGrath
Dump
mysqldump db_name table_name | gzip > table_name.sql.gz
Restore
gunzip < table_name.sql.gz | mysql -u username -p db_name
| MySQL | 6,682,916 | 559 |
Consider:
./mysqladmin -u root -p** '_redacted_'
Output (including typing the password):
Enter password:
mysqladmin: connect to server at 'localhost' failed error:
'Access denied for user 'root'@'localhost' (using password: YES)'
How can I fix this?
| All solutions I found were much more complex than necessary and none worked for me. Here is the solution that solved my problem. There isn't any need to restart mysqld or start it with special privileges.
sudo mysql
-- for MySQL
ALTER USER 'root'@'localhost' IDENTIFIED WITH mysql_native_password BY 'root';
-- for MariaDB
ALTER USER 'root'@'localhost' IDENTIFIED VIA mysql_native_password USING PASSWORD('root');
With a single query we are changing the auth_plugin to mysql_native_password and setting the root password to root (feel free to change it in the query).
Now you should be able to log in with root. More information can be found in MySQL documentation or MariaDB documentation.
(Exit the MySQL console with Ctrl + D or by typing exit.)
| MySQL | 41,645,309 | 557 |
How can I best write a query that selects 10 rows randomly from a total of 600k?
| A great post handling several cases, from simple, to gaps, to non-uniform with gaps.
http://jan.kneschke.de/projects/mysql/order-by-rand/
For most general case, here is how you do it:
SELECT name
FROM random AS r1 JOIN
(SELECT CEIL(RAND() *
(SELECT MAX(id)
FROM random)) AS id)
AS r2
WHERE r1.id >= r2.id
ORDER BY r1.id ASC
LIMIT 1
This supposes that the distribution of ids is equal, and that there can be gaps in the id list. See the article for more advanced examples
| MySQL | 4,329,396 | 556 |
Is there any query/way to show the last queries executed on ALL servers?
| For those blessed with MySQL >= 5.1.12, you can control this option globally at runtime:
Execute SET GLOBAL log_output = 'TABLE';
Execute SET GLOBAL general_log = 'ON';
Take a look at the table mysql.general_log
If you prefer to output to a file instead of a table:
SET GLOBAL log_output = "FILE"; the default.
SET GLOBAL general_log_file = "/path/to/your/logfile.log";
SET GLOBAL general_log = 'ON';
I prefer this method to editing .cnf files because:
you're not editing the my.cnf file and potentially permanently turning on logging
you're not fishing around the filesystem looking for the query log - or even worse, distracted by the need for the perfect destination. /var/log /var/data/log /opt /home/mysql_savior/var
You don't have to restart the server and interrupt any current connections to it.
restarting the server leaves you where you started (log is by default still off)
For more information, see
MySQL 5.1 Reference Manual - Server System Variables - general_log
| MySQL | 650,238 | 553 |
In another question I posted someone told me that there is a difference between:
@variable
and:
variable
in MySQL. He also mentioned how MSSQL has batch scope and MySQL has session scope. Can someone elaborate on this for me?
| MySQL has a concept of user-defined variables.
They are loosely typed variables that may be initialized somewhere in a session and keep their value until the session ends.
They are prepended with an @ sign, like this: @var
You can initialize this variable with a SET statement or inside a query:
SET @var = 1
SELECT @var2 := 2
When you develop a stored procedure in MySQL, you can pass the input parameters and declare the local variables:
DELIMITER //
CREATE PROCEDURE prc_test (var INT)
BEGIN
DECLARE var2 INT;
SET var2 = 1;
SELECT var2;
END;
//
DELIMITER ;
These variables are not prepended with any prefixes.
The difference between a procedure variable and a session-specific user-defined variable is that a procedure variable is reinitialized to NULL each time the procedure is called, while the session-specific variable is not:
CREATE PROCEDURE prc_test ()
BEGIN
DECLARE var2 INT DEFAULT 1;
SET var2 = var2 + 1;
SET @var2 = @var2 + 1;
SELECT var2, @var2;
END;
SET @var2 = 1;
CALL prc_test();
var2 @var2
--- ---
2 2
CALL prc_test();
var2 @var2
--- ---
2 3
CALL prc_test();
var2 @var2
--- ---
2 4
As you can see, var2 (procedure variable) is reinitialized each time the procedure is called, while @var2 (session-specific variable) is not.
(In addition to user-defined variables, MySQL also has some predefined "system variables", which may be "global variables" such as @@global.port or "session variables" such as @@session.sql_mode; these "session variables" are unrelated to session-specific user-defined variables.)
| MySQL | 1,009,954 | 551 |
I am learning MySQL and tried using a LOAD DATA clause. When I used it as below:
LOAD DATA INFILE "text.txt" INTO table mytable;
I got the following error:
The MySQL server is running with the --secure-file-priv option so it cannot execute this statement
How do I tackle this error?
I have checked another question on the same error message, but still can’t find a solution.
I am using MySQL 5.6
| It's working as intended. Your MySQL server has been started with --secure-file-priv option which limits from which directories you can load files using LOAD DATA INFILE.
Use SHOW VARIABLES LIKE "secure_file_priv"; to see the directory that has been configured.
You have two options:
Move your file to the directory specified by secure-file-priv.
Disable secure-file-priv. This must be removed from startup and cannot be modified dynamically. To do this check your MySQL start up parameters (depending on platform) and my.ini.
| MySQL | 32,737,478 | 550 |
When I open XAMPP and click the start MySQL button, it gives me an error.
I had started it just before, but now it isn't working.
12:19:12 PM [mysql] Attempting to start MySQL app...
12:19:12 PM [mysql] Status change detected: running
12:19:13 PM [mysql] Status change detected: stopped
12:19:13 PM [mysql] Error: MySQL shutdown unexpectedly.
12:19:13 PM [mysql] This may be due to a blocked port, missing dependencies,
12:19:13 PM [mysql] improper privileges, a crash, or a shutdown by another method
12:19:13 PM [mysql] Press the Logs button to view error logs and check
12:19:13 PM [mysql] the Windows Event Viewer for more clues
12:19:13 PM [mysql] If you need more help, copy and post this
12:19:13 PM [mysql] entire log window on the forums
Here is the contents of the error log:
2013-08-02 12:19:12 4536 [Note] Plugin 'FEDERATED' is disabled.
2013-08-02 12:19:12 f64 InnoDB: Warning: Using innodb_additional_mem_pool_size is DEPRECATED. This option may be removed in future releases, together with the option innodb_use_sys_malloc and with the InnoDB's internal memory allocator.
2013-08-02 12:19:12 4536 [Note] InnoDB: The InnoDB memory heap is disabled
2013-08-02 12:19:12 4536 [Note] InnoDB: Mutexes and rw_locks use Windows interlocked functions
2013-08-02 12:19:12 4536 [Note] InnoDB: Compressed tables use zlib 1.2.3
2013-08-02 12:19:12 4536 [Note] InnoDB: Not using CPU crc32 instructions
2013-08-02 12:19:12 4536 [Note] InnoDB: Initializing buffer pool, size = 16.0M
2013-08-02 12:19:12 4536 [Note] InnoDB: Completed initialization of buffer pool
2013-08-02 12:19:12 4536 [Note] InnoDB: Highest supported file format is Barracuda.
2013-08-02 12:19:12 4536 [Note] InnoDB: The log sequence numbers 0 and 0 in ibdata files do not match the log sequence number 1616798 in the ib_logfiles!
2013-08-02 12:19:12 4536 [Note] InnoDB: Database was not shutdown normally!
2013-08-02 12:19:12 4536 [Note] InnoDB: Starting crash recovery.
2013-08-02 12:19:12 4536 [Note] InnoDB: Reading tablespace information from the .ibd files...
The most important error message is here:
2013-08-02 12:19:12 4536 [ERROR] InnoDB:
Attempted to open a previously opened tablespace.
Previous tablespace mysql/innodb_table_stats uses space ID: 1 at filepath: .\mysql\innodb_table_stats.ibd.
Cannot open tablespace xat/payments which uses space ID: 1 at filepath: .\xat\payments.ibd
The rest of the log:
InnoDB: Error: could not open single-table tablespace file .\xat\payments.ibd
InnoDB: We do not continue the crash recovery, because the table may become
InnoDB: corrupt if we cannot apply the log records in the InnoDB log to it.
InnoDB: To fix the problem and start mysqld:
InnoDB: 1) If there is a permission problem in the file and mysqld cannot
InnoDB: open the file, you should modify the permissions.
InnoDB: 2) If the table is not needed, or you can restore it from a backup,
InnoDB: then you can remove the .ibd file, and InnoDB will do a normal
InnoDB: crash recovery and ignore that table.
InnoDB: 3) If the file system or the disk is broken, and you cannot remove
InnoDB: the .ibd file, you can set innodb_force_recovery > 0 in my.cnf
InnoDB: and force InnoDB to continue crash recovery here.
What is the cause of these errors and how can I rectify them?
| Important: do not delete the ibdata1 file. You could destroy all your databases.
Instead, first try using the MySQL backup folder which is included with XAMPP. So do next steps:
Rename folder mysql/data to mysql/data_old
Make a copy of mysql/backup folder and name it as mysql/data
Copy all your database folders from mysql/data_old into mysql/data (except mysql, performance_schema, and phpmyadmin folders)
Copy mysql/data_old/ibdata1 file into mysql/data folder
Start MySQL from XAMPP control panel
And, voilà!
As mentioned by some users, this is an emergency solution, not a permanent one. After recovering your data is strongly recommended to back it up, and reinstalling XAMPP, because the failure is related to a malfunction from some of the files of XAMPP, not the databases.
| MySQL | 18,022,809 | 544 |
First let me mention that I've gone through many suggested questions and found no relevent answer. Here is what I'm doing.
I'm connected to my Amazon EC2 instance. I can login with MySQL root with this command:
mysql -u root -p
Then I created a new user bill with host %
CREATE USER 'bill'@'%' IDENTIFIED BY 'passpass';
Granted all the privileges to user bill:
grant all privileges on *.* to 'bill'@'%' with grant option;
Then I exit from root user and try to login with bill:
mysql -u bill -p
entered the correct password and got this error:
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
| You probably have an anonymous user ''@'localhost' or ''@'127.0.0.1'.
As per the manual:
When multiple matches are possible, the server must determine which of
them to use. It resolves this issue as follows: (...)
When a client attempts to connect, the server looks through the rows [of table mysql.user] in sorted order.
The server uses the first row that matches the client host name and user name.
(...)
The server uses sorting rules that order rows with the most-specific Host values first.
Literal host names [such as 'localhost'] and IP addresses are the most specific.
Therefore such an anonymous user would "mask" any other user like '[any_username]'@'%' when connecting from localhost.
'bill'@'localhost' does match 'bill'@'%', but would match (e.g.) ''@'localhost' beforehands.
The recommended solution is to drop this anonymous user (this is usually a good thing to do anyways).
Below edits are mostly irrelevant to the main question. These are only meant to answer some questions raised in other comments within this thread.
Edit 1
Authenticating as 'bill'@'%' through a socket.
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass --socket=/tmp/mysql-5.5.sock
Welcome to the MySQL monitor (...)
mysql> SELECT user, host FROM mysql.user;
+------+-----------+
| user | host |
+------+-----------+
| bill | % |
| root | 127.0.0.1 |
| root | ::1 |
| root | localhost |
+------+-----------+
4 rows in set (0.00 sec)
mysql> SELECT USER(), CURRENT_USER();
+----------------+----------------+
| USER() | CURRENT_USER() |
+----------------+----------------+
| bill@localhost | bill@% |
+----------------+----------------+
1 row in set (0.02 sec)
mysql> SHOW VARIABLES LIKE 'skip_networking';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| skip_networking | ON |
+-----------------+-------+
1 row in set (0.00 sec)
Edit 2
Exact same setup, except I re-activated networking, and I now create an anonymous user ''@'localhost'.
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql
Welcome to the MySQL monitor (...)
mysql> CREATE USER ''@'localhost' IDENTIFIED BY 'anotherpass';
Query OK, 0 rows affected (0.00 sec)
mysql> Bye
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass \
--socket=/tmp/mysql-5.5.sock
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass \
-h127.0.0.1 --protocol=TCP
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -ppass \
-hlocalhost --protocol=TCP
ERROR 1045 (28000): Access denied for user 'bill'@'localhost' (using password: YES)
Edit 3
Same situation as in edit 2, now providing the anonymous user's password.
root@myhost:/home/mysql-5.5.16-linux2.6-x86_64# ./mysql -ubill -panotherpass -hlocalhost
Welcome to the MySQL monitor (...)
mysql> SELECT USER(), CURRENT_USER();
+----------------+----------------+
| USER() | CURRENT_USER() |
+----------------+----------------+
| bill@localhost | @localhost |
+----------------+----------------+
1 row in set (0.01 sec)
Conclusion 1, from edit 1: One can authenticate as 'bill'@'%'through a socket.
Conclusion 2, from edit 2: Whether one connects through TCP or through a socket has no impact on the authentication process (except one cannot connect as anyone else but 'something'@'localhost' through a socket, obviously).
Conclusion 3, from edit 3: Although I specified -ubill, I have been granted access as an anonymous user. This is because of the "sorting rules" advised above. Notice that in most default installations, a no-password, anonymous user exists (and should be secured/removed).
| MySQL | 10,299,148 | 542 |
How can I convert entire MySQL database character-set to UTF-8 and collation to UTF-8?
| Use the ALTER DATABASE and ALTER TABLE commands.
ALTER DATABASE databasename CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
ALTER TABLE tablename CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
Or if you're still on MySQL 5.5.2 or older which didn't support 4-byte UTF-8, use utf8 instead of utf8mb4:
ALTER DATABASE databasename CHARACTER SET utf8 COLLATE utf8_unicode_ci;
ALTER TABLE tablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci;
| MySQL | 6,115,612 | 542 |
I'm creating a form for sending private messages and want to set the maxlength value of a textarea appropriate to the max length of a text field in my MySQL database table. How many characters can a type text field store?
If a lot, would I be able to specify length in the database text type field as I would with varchar?
| See for maximum numbers:
http://dev.mysql.com/doc/refman/5.0/en/storage-requirements.html
TINYBLOB, TINYTEXT L + 1 bytes, where L < 2^8 (255 Bytes)
BLOB, TEXT L + 2 bytes, where L < 2^16 (64 Kilobytes)
MEDIUMBLOB, MEDIUMTEXT L + 3 bytes, where L < 2^24 (16 Megabytes)
LONGBLOB, LONGTEXT L + 4 bytes, where L < 2^32 (4 Gigabytes)
L is the number of bytes in your text field. So the maximum number of chars for text is 216-1 (using single-byte characters). Means 65 535 chars(using single-byte characters).
UTF-8/MultiByte encoding: using MultiByte encoding each character might consume more than 1 byte of space. For UTF-8 space consumption is between 1 to 4 bytes per char.
| MySQL | 6,766,781 | 540 |
I am having some problems when trying to install mysql2 gem for Rails. When I try to install it by running bundle install or gem install mysql2 it gives me the following error:
Error installing mysql2: ERROR: Failed to build gem native extension.
How can I fix this and successfully install mysql2?
| On Ubuntu/Debian and other distributions using aptitude:
sudo apt-get install libmysql-ruby libmysqlclient-dev
Package libmysql-ruby has been phased out and replaced by ruby-mysql. This is where I found the solution.
If the above command doesn't work because libmysql-ruby cannot be found, the following should be sufficient:
sudo apt-get install libmysqlclient-dev
On Red Hat/CentOS and other distributions using yum:
sudo yum install mysql-devel
On Mac OS X with Homebrew:
brew install mysql
| MySQL | 3,608,287 | 535 |
We offer a platform for video- and audio-clips, photos and vector-grafics. We started with MySQL as the database backend and recently included MongoDB for storing all meta-information of the files, because MongoDB better fits the requirements. For example: photos may have Exif information, videos may have audio-tracks where we to want to store the meta-information of, too. Videos and vector-graphics don't share any common meta-information, etc. so I know, that MongoDB is perfect to store this unstructured data and keep it searchable.
However, we continue developing our platform and adding features. Now one of the next steps will be providing a forum for our users. The question that now arises is: use the MySQL database, which would be a good choice for storing forums and forum-posts, etc. or use MongoDB for this, too?
So the question is: when to use MongoDB and when to use a RDBMS. What would you take, mongoDB or MySQL, if you had the choice and why would you take it?
| In NoSQL: If Only It Was That Easy, the author writes about MongoDB:
MongoDB is not a key/value store, it’s quite a bit more. It’s definitely not a RDBMS either. I haven’t used MongoDB in production, but I have used it a little building a test app and it is a very cool piece of kit. It seems to be very performant and either has, or will have soon, fault tolerance and auto-sharding (aka it will scale). I think Mongo might be the closest thing to a RDBMS replacement that I’ve seen so far. It won’t work for all data sets and access patterns, but it’s built for your typical CRUD stuff. Storing what is essentially a huge hash, and being able to select on any of those keys, is what most people use a relational database for. If your DB is 3NF and you don’t do any joins (you’re just selecting a bunch of tables and putting all the objects together, AKA what most people do in a web app), MongoDB would probably kick ass for you.
Then, in the conclusion:
The real thing to point out is that if you are being held back from making something super awesome because you can’t choose a database, you are doing it wrong. If you know mysql, just use it. Optimize when you actually need to. Use it like a k/v store, use it like a rdbms, but for god sake, build your killer app! None of this will matter to most apps. Facebook still uses MySQL, a lot. Wikipedia uses MySQL, a lot. FriendFeed uses MySQL, a lot. NoSQL is a great tool, but it’s certainly not going to be your competitive edge, it’s not going to make your app hot, and most of all, your users won’t care about any of this.
What am I going to build my next app on? Probably Postgres. Will I use NoSQL? Maybe. I might also use Hadoop and Hive. I might keep everything in flat files. Maybe I’ll start hacking on Maglev. I’ll use whatever is best for the job. If I need reporting, I won’t be using any NoSQL. If I need caching, I’ll probably use Tokyo Tyrant. If I need ACIDity, I won’t use NoSQL. If I need a ton of counters, I’ll use Redis. If I need transactions, I’ll use Postgres. If I have a ton of a single type of documents, I’ll probably use Mongo. If I need to write 1 billion objects a day, I’d probably use Voldemort. If I need full text search, I’d probably use Solr. If I need full text search of volatile data, I’d probably use Sphinx.
I like this article, I find it very informative, it gives a good overview of the NoSQL landscape and hype. But, and that's the most important part, it really helps to ask yourself the right questions when it comes to choose between RDBMS and NoSQL. Worth the read IMHO.
Alternate link to article
| MySQL | 1,476,295 | 531 |
MySQL 5.0.45
What is the syntax to alter a table to allow a column to be null, alternately what's wrong with this:
ALTER mytable MODIFY mycolumn varchar(255) null;
I interpreted the manual as just run the above and it would recreate the column, this time allowing null. The server is telling me I have syntactical errors. I just don't see them.
| You want the following:
ALTER TABLE mytable MODIFY mycolumn VARCHAR(255);
Columns are nullable by default. As long as the column is not declared UNIQUE or NOT NULL, there shouldn't be any problems.
| MySQL | 212,939 | 529 |
What is the difference between utf8mb4 and utf8 charsets in MySQL?
I already know about ASCII, UTF-8, UTF-16 and UTF-32 encodings;
but I'm curious to know whats the difference of utf8mb4 group of encodings with other encoding types defined in MySQL Server.
Are there any special benefits/proposes of using utf8mb4 rather than utf8?
| UTF-8 is a variable-length encoding. In the case of UTF-8, this means that storing one code point requires one to four bytes. However, MySQL's encoding called "utf8" (alias of "utf8mb3") only stores a maximum of three bytes per code point.
So the character set "utf8"/"utf8mb3" cannot store all Unicode code points: it only supports the range 0x000 to 0xFFFF, which is called the "Basic Multilingual Plane".
See also Comparison of Unicode encodings.
This is what (a previous version of the same page at) the MySQL documentation has to say about it:
The character set named utf8[/utf8mb3] uses a maximum of three bytes per character and contains only BMP characters. As of MySQL 5.5.3, the utf8mb4 character set uses a maximum of four bytes per character supports supplemental characters:
For a BMP character, utf8[/utf8mb3] and utf8mb4 have identical storage characteristics: same code values, same encoding, same length.
For a supplementary character, utf8[/utf8mb3] cannot store the character at all, while utf8mb4 requires four bytes to store it. Since utf8[/utf8mb3] cannot store the character at all, you do not have any supplementary characters in utf8[/utf8mb3] columns and you need not worry about converting characters or losing data when upgrading utf8[/utf8mb3] data from older versions of MySQL.
So if you want your column to support storing characters lying outside the BMP (and you usually want to), such as emoji, use "utf8mb4". See also What are the most common non-BMP Unicode characters in actual use?.
| MySQL | 30,074,492 | 522 |
I have a table with a primary key that is a varchar(255). Some cases have arisen where 255 characters isn't enough. I tried changing the field to a text, but I get the following error:
BLOB/TEXT column 'message_id' used in key specification without a key length
how can I fix this?
edit: I should also point out this table has a composite primary key with multiple columns.
| The error happens because MySQL can index only the first N chars of a BLOB or TEXT column. So The error mainly happens when there is a field/column type of TEXT or BLOB or those belong to TEXT or BLOB types such as TINYBLOB, MEDIUMBLOB, LONGBLOB, TINYTEXT, MEDIUMTEXT, and LONGTEXT that you try to make a primary key or index. With full BLOB or TEXT without the length value, MySQL is unable to guarantee the uniqueness of the column as it’s of variable and dynamic size. So, when using BLOB or TEXT types as an index, the value of N must be supplied so that MySQL can determine the key length. However, MySQL doesn’t support a key length limit on TEXT or BLOB. TEXT(88) simply won’t work.
The error will also pop up when you try to convert a table column from non-TEXT and non-BLOB type such as VARCHAR and ENUM into TEXT or BLOB type, with the column already been defined as unique constraints or index. The Alter Table SQL command will fail.
The solution to the problem is to remove the TEXT or BLOB column from the index or unique constraint or set another field as primary key. If you can't do that, and wanting to place a limit on the TEXT or BLOB column, try to use VARCHAR type and place a limit of length on it. By default, VARCHAR is limited to a maximum of 255 characters and its limit must be specified implicitly within a bracket right after its declaration, i.e VARCHAR(200) will limit it to 200 characters long only.
Sometimes, even though you don’t use TEXT or BLOB related type in your table, the Error 1170 may also appear. It happens in a situation such as when you specify VARCHAR column as primary key, but wrongly set its length or characters size. VARCHAR can only accepts up to 256 characters, so anything such as VARCHAR(512) will force MySQL to auto-convert the VARCHAR(512) to a SMALLTEXT datatype, which subsequently fails with error 1170 on key length if the column is used as primary key or unique or non-unique index. To solve this problem, specify a figure less than 256 as the size for VARCHAR field.
Reference: MySQL Error 1170 (42000): BLOB/TEXT Column Used in Key Specification Without a Key Length
| MySQL | 1,827,063 | 521 |
I want to copy a live production database into my local development database. Is there a way to do this without locking the production database?
I'm currently using:
mysqldump -u root --password=xxx -h xxx my_db1 | mysql -u root --password=xxx -h localhost my_db1
But it's locking each table as it runs.
| Does the --lock-tables=false option work?
According to the man page, if you are dumping InnoDB tables you can use the --single-transaction option:
--lock-tables, -l
Lock all tables before dumping them. The tables are locked with READ
LOCAL to allow concurrent inserts in the case of MyISAM tables. For
transactional tables such as InnoDB and BDB, --single-transaction is
a much better option, because it does not need to lock the tables at
all.
For innodb DB:
mysqldump --single-transaction=TRUE -u username -p DB
| MySQL | 104,612 | 520 |
How can I see the list of the stored procedures or stored functions in mysql command line like SHOW TABLES; or SHOW DATABASES; commands.
| SHOW PROCEDURE STATUS;
SHOW FUNCTION STATUS;
| MySQL | 733,349 | 518 |
Is there a MySQL command to locate the my.cnf configuration file, similar to how PHP's phpinfo() locates its php.ini?
| There is no internal MySQL command to trace this, it's a little too abstract. The file might be in 5 (or more?) locations, and they would all be valid because they load cascading.
/etc/my.cnf
/etc/mysql/my.cnf
$MYSQL_HOME/my.cnf
[datadir]/my.cnf
~/.my.cnf
Those are the default locations MySQL looks at. If it finds more than one, it will load each of them & values override each other (in the listed order, I think). Also, the --defaults-file parameter can override the whole thing, so... basically, it's a huge pain in the butt.
But thanks to it being so confusing, there's a good chance it's just in /etc/my.cnf.
(If you just want to see the values: SHOW VARIABLES, but you'll need the permissions to do so.)
Run mysql --help and you will see:
Default options are read from the following files in the given order: /etc/my.cnf /etc/mysql/my.cnf ~/.my.cnf
| MySQL | 2,482,234 | 514 |
I try to connect MySQL database with Java using connector 8.0.11. Everything seems to be OK, but I get this exception:
Exception in thread "main" java.sql.SQLNonTransientConnectionException: Public Key Retrieval is not allowed at
com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:108) at
com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:95) at
com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) at
com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:862) at
com.mysql.cj.jdbc.ConnectionImpl.(ConnectionImpl.java:444) at
com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:230) at
com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:226) at
com.mysql.cj.jdbc.MysqlDataSource.getConnection(MysqlDataSource.java:438) at
com.mysql.cj.jdbc.MysqlDataSource.getConnection(MysqlDataSource.java:146) at
com.mysql.cj.jdbc.MysqlDataSource.getConnection(MysqlDataSource.java:119) at
ConnectionManager.getConnection(ConnectionManager.java:28) at
Main.main(Main.java:8)
Here is my Connection Manager class:
public class ConnectionManager {
public static final String serverTimeZone = "UTC";
public static final String serverName = "localhost";
public static final String databaseName ="biblioteka";
public static final int portNumber = 3306;
public static final String user = "anyroot";
public static final String password = "anyroot";
public static Connection getConnection() throws SQLException {
MysqlDataSource dataSource = new MysqlDataSource();
dataSource.setUseSSL( false );
dataSource.setServerTimezone( serverTimeZone );
dataSource.setServerName( serverName );
dataSource.setDatabaseName( databaseName );
dataSource.setPortNumber( portNumber );
dataSource.setUser( user );
dataSource.setPassword( password );
return dataSource.getConnection();
}
}
| You should add client option to your mysql-connector allowPublicKeyRetrieval=true to allow the client to automatically request the public key from the server. Note that allowPublicKeyRetrieval=True could allow a malicious proxy to perform a MITM attack to get the plaintext password, so it is False by default and must be explicitly enabled.
See MySQL .NET Connection String Options
you could also try adding useSSL=false when you use it for testing/develop purposes
example:
jdbc:mysql://localhost:3306/db?allowPublicKeyRetrieval=true&useSSL=false
| MySQL | 50,379,839 | 510 |
I have upgraded my system and have installed MySql 5.7.9 with php for a web application I am working on. I have a query that is dynamically created, and when run in older versions of MySQL it works fine. Since upgrading to 5.7 I get this error:
Expression #1 of SELECT list is not in GROUP BY clause and contains
non-aggregated column 'support_desk.mod_users_groups.group_id' which is
not functionally dependent on columns in GROUP BY clause; this is
incompatible with sql_mode=only_full_group_by
Note the manual page for Mysql 5.7 on the topic of Server SQL Modes.
This is the query that is giving me trouble:
SELECT mod_users_groups.group_id AS 'value',
group_name AS 'text'
FROM mod_users_groups
LEFT JOIN mod_users_data ON mod_users_groups.group_id = mod_users_data.group_id
WHERE mod_users_groups.active = 1
AND mod_users_groups.department_id = 1
AND mod_users_groups.manage_work_orders = 1
AND group_name != 'root'
AND group_name != 'superuser'
GROUP BY group_name
HAVING COUNT(`user_id`) > 0
ORDER BY group_name
I don't understand only_full_group_by enough to figure out what I need to do to fix the query. Can I just turn off the only_full_group_by option, or is there something else I need to do?
| I would just add group_id to the GROUP BY.
When SELECTing a column that is not part of the GROUP BY there could be multiple values for that column within the groups, but there will only be space for a single value in the results. So, the database usually needs to be told exactly how to make those multiple values into one value. Commonly, this is done with an aggregate function like COUNT(), SUM(), MAX() etc... I say usually because most other popular database systems insist on this. However, in MySQL prior to version 5.7 the default behaviour has been more forgiving because it will not complain and then arbitrarily choose any value! It also has an ANY_VALUE() function that could be used as another solution to this question if you really needed the same behaviour as before. This flexibility comes at a cost because it is non-deterministic, so I would not recommend it unless you have a very good reason for needing it. MySQL are now turning on the only_full_group_by setting by default for good reasons, so it's best to get used to it and make your queries comply with it.
So why my simple answer above? I've made a couple of assumptions:
1) the group_id is unique. Seems reasonable, it is an 'ID' after all.
2) the group_name is also unique. This may not be such a reasonable assumption. If this is not the case and you have some duplicate group_names and you then follow my advice to add group_id to the GROUP BY, you may find that you now get more results than before because the groups with the same name will now have separate rows in the results. To me, this would be better than having these duplicate groups hidden because the database has quietly selected a value arbitrarily!
It's also good practice to qualify all the columns with their table name or alias when there's more than one table involved...
SELECT
g.group_id AS 'value',
g.group_name AS 'text'
FROM mod_users_groups g
LEFT JOIN mod_users_data d ON g.group_id = d.group_id
WHERE g.active = 1
AND g.department_id = 1
AND g.manage_work_orders = 1
AND g.group_name != 'root'
AND g.group_name != 'superuser'
GROUP BY
g.group_name,
g.group_id
HAVING COUNT(d.user_id) > 0
ORDER BY g.group_name
| MySQL | 34,115,174 | 509 |
I am running this query on MySQL
SELECT ID FROM (
SELECT ID, msisdn
FROM (
SELECT * FROM TT2
)
);
and it is giving this error:
Every derived table must have its own alias.
What's causing this error?
| Every derived table (AKA sub-query) must indeed have an alias. I.e. each query in brackets must be given an alias (AS whatever), which can the be used to refer to it in the rest of the outer query.
SELECT ID FROM (
SELECT ID, msisdn FROM (
SELECT * FROM TT2
) AS T
) AS T
In your case, of course, the entire query could be replaced with:
SELECT ID FROM TT2
| MySQL | 1,888,779 | 507 |
I'm just getting started learning SQLite. It would be nice to be able to see the details for a table, like MySQL's DESCRIBE [table]. PRAGMA table_info [table] isn't good enough, as it only has basic information (for example, it doesn't show if a column is a field of some sort or not). Does SQLite have a way to do this?
| The SQLite command line utility has a .schema TABLENAME command that shows you the create statements.
| MySQL | 3,330,435 | 504 |
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