instruction: list all flights leaving DENVER between 2000 and 2100
response: SELECT DISTINCT flight_id FROM flight WHERE ((departure_time <= 2100 AND departure_time >= 2000) 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 = 'DENVER')))