find the cheapest flight from denver to atlanta with a stopover in pittsburgh 
return flights ;return cheapness of  flights ;return #1 from  denver ;return #3 to atlanta ;return #4 with a stopover in pittsburgh ;return #5 sorted by #2 ;return the  cheapest #6
['select', 'select', 'filter', 'filter', 'filter', 'sort', 'project']