instruction: i would like to book a flight from DENVER to PITTSBURGH on 7 15 i'd like it to be the cheapest flight
response: SELECT DISTINCT flight.flight_id FROM airport_service AS AIRPORT_SERVICE_0, airport_service AS AIRPORT_SERVICE_1, airport_service AS AIRPORT_SERVICE_2, airport_service AS AIRPORT_SERVICE_3, city AS CITY_0, city AS CITY_1, city AS CITY_2, city AS CITY_3, date_day AS DATE_DAY_0, date_day AS DATE_DAY_1, days AS DAYS_0, days AS DAYS_1, fare, fare_basis, flight, flight_fare WHERE ((((CITY_3.city_code = AIRPORT_SERVICE_3.city_code AND CITY_3.city_name = 'PITTSBURGH' AND DATE_DAY_1.day_number = 15 AND DATE_DAY_1.month_number = 7 AND DATE_DAY_1.year = 1991 AND DAYS_1.day_name = DATE_DAY_1.day_name AND fare_basis.basis_days = DAYS_1.days_code AND fare.fare_basis_code = fare_basis.fare_basis_code AND fare.to_airport = AIRPORT_SERVICE_3.airport_code) AND CITY_2.city_code = AIRPORT_SERVICE_2.city_code AND CITY_2.city_name = 'DENVER' AND fare.from_airport = AIRPORT_SERVICE_2.airport_code) AND DATE_DAY_0.day_number = 15 AND DATE_DAY_0.month_number = 7 AND DATE_DAY_0.year = 1991 AND DAYS_0.day_name = DATE_DAY_0.day_name AND fare.one_direction_cost = (SELECT MIN(FAREalias1.one_direction_cost) FROM airport_service AS AIRPORT_SERVICEalias4, airport_service AS AIRPORT_SERVICEalias5, city AS CITYalias4, city AS CITYalias5, date_day AS DATE_DAYalias2, days AS DAYSalias2, fare AS FAREalias1, fare_basis AS FARE_BASISalias1 WHERE (CITYalias5.city_code = AIRPORT_SERVICEalias5.city_code AND CITYalias5.city_name = 'PITTSBURGH' AND DATE_DAYalias2.day_number = 15 AND DATE_DAYalias2.month_number = 7 AND DATE_DAYalias2.year = 1991 AND DAYSalias2.day_name = DATE_DAYalias2.day_name AND FARE_BASISalias1.basis_days = DAYSalias2.days_code AND FAREalias1.fare_basis_code = FARE_BASISalias1.fare_basis_code AND FAREalias1.to_airport = AIRPORT_SERVICEalias5.airport_code) AND CITYalias4.city_code = AIRPORT_SERVICEalias4.city_code AND CITYalias4.city_name = 'DENVER' AND FAREalias1.from_airport = AIRPORT_SERVICEalias4.airport_code) AND flight_fare.fare_id = fare.fare_id AND flight.flight_days = DAYS_0.days_code AND flight.flight_id = flight_fare.flight_id) AND CITY_1.city_code = AIRPORT_SERVICE_1.city_code AND CITY_1.city_name = 'PITTSBURGH' AND flight.to_airport = AIRPORT_SERVICE_1.airport_code) AND CITY_0.city_code = AIRPORT_SERVICE_0.city_code AND CITY_0.city_name = 'DENVER' AND flight.from_airport = AIRPORT_SERVICE_0.airport_code