YX ให้บริการบัลติมอร์หรือไม่ ไม่จำเป็นต้องใช้ตารางทั้งหมด
does YX serve BALTIMOREIt is not neccessary to use all the tables.
 class_of_service:  booking_class [ varchar]  rank [ int]  class_description [ text] 
SELECT DISTINCT flight.flight_id FROM airport_service AS AIRPORT_SERVICE_0, airport_service AS AIRPORT_SERVICE_1, city AS CITY_0, city AS CITY_1, flight WHERE ((flight.to_airport = AIRPORT_SERVICE_0.airport_code AND CITY_0.city_code = AIRPORT_SERVICE_0.city_code AND CITY_0.city_name = 'BALTIMORE') OR (flight.from_airport = AIRPORT_SERVICE_1.airport_code AND CITY_1.city_code = AIRPORT_SERVICE_1.city_code AND CITY_1.city_name = 'BALTIMORE')) AND flight.airline_code = 'YX'