i want a flight originating in denver going to pittsburgh and atlanta in either order 
return flights ;return #1 from  denver ;return #2 to pittsburgh ;return #3 to atlanta ;return #2 to atlanta ;return #5 to pittsburgh ;return #4 ,  #6
['select', 'filter', 'filter', 'filter', 'filter', 'filter', 'union']