instruction: what are the flights for UA on 9 20 in the morning
response: SELECT DISTINCT flight.flight_id FROM date_day, days, flight WHERE (date_day.day_number = 20 AND date_day.month_number = 9 AND date_day.year = 1991 AND days.day_name = date_day.day_name AND flight.airline_code = 'UA' AND flight.flight_days = days.days_code) AND flight.departure_time BETWEEN 0 AND 1200