instruction: please list all flights that leave DENVER before 1200 on NW
response: SELECT DISTINCT flight.flight_id FROM airport_service, city, flight WHERE (city.city_code = airport_service.city_code AND city.city_name = 'DENVER' AND flight.departure_time < 1200 AND flight.from_airport = airport_service.airport_code) AND flight.airline_code = 'NW'