instruction: what flights leave PITTSBURGH for DENVER on monday night
response: SELECT DISTINCT flight_id FROM flight WHERE (((((departure_time <= 2400 AND departure_time >= 1800) AND flight_days IN (SELECT DAYSalias0.days_code FROM days AS DAYSalias0 WHERE DAYSalias0.day_name IN (SELECT DATE_DAYalias0.day_name FROM date_day AS DATE_DAYalias0 WHERE DATE_DAYalias0.day_number = 21 AND DATE_DAYalias0.month_number = 2 AND DATE_DAYalias0.year = 1991))) OR (departure_time <= 600 AND flight_days IN (SELECT DAYSalias1.days_code FROM days AS DAYSalias1 WHERE DAYSalias1.day_name IN (SELECT DATE_DAYalias1.day_name FROM date_day AS DATE_DAYalias1 WHERE DATE_DAYalias1.day_number = 21 AND DATE_DAYalias1.month_number = 2 AND DATE_DAYalias1.year = 1991)))) AND to_airport IN (SELECT AIRPORT_SERVICEalias1.airport_code FROM airport_service AS AIRPORT_SERVICEalias1 WHERE AIRPORT_SERVICEalias1.city_code IN (SELECT CITYalias1.city_code FROM city AS CITYalias1 WHERE CITYalias1.city_name = 'DENVER'))) AND from_airport IN (SELECT AIRPORT_SERVICEalias0.airport_code FROM airport_service AS AIRPORT_SERVICEalias0 WHERE AIRPORT_SERVICEalias0.city_code IN (SELECT CITYalias0.city_code FROM city AS CITYalias0 WHERE CITYalias0.city_name = 'PITTSBURGH')))