instruction: what flights are available from PITTSBURGH to OAK airport
response: SELECT DISTINCT flight.flight_id FROM airport, airport_service, city, flight WHERE airport.airport_code = 'OAK' AND city.city_code = airport_service.city_code AND city.city_name = 'PITTSBURGH' AND flight.from_airport = airport_service.airport_code AND flight.to_airport = airport.airport_code