please list all flights on united airlines and northwest airlines that go into denver and all flights on northwest airlines and united airlines that fly out of denver 
return flights ;return #1 on northwest airlines   ;return #1 on united airlines    ;return #2 that go into denver ;return #2 that fly out of denver ;return #3 that go into denver ;return #3 that fly out of denver ;return #4 ,  #6 ,   #5 , #7
['select', 'filter', 'filter', 'filter', 'filter', 'filter', 'filter', 'union']