AA ให้บริการเมืองใดบ้างไม่จำเป็นต้องใช้ตารางทั้งหมด
what are the cities that AA servesIt is not neccessary to use all the tables.
 airport:  airport_code [ varchar]  airport_name [ text]  airport_location [ text]  state_code [ varchar]  country_name [ varchar]  time_zone_code [ varchar]  minimum_connect_time [ int] 
SELECT DISTINCT city.city_code FROM airport_service, city, flight WHERE city.city_code = airport_service.city_code AND flight.airline_code = 'AA' AND flight.from_airport = airport_service.airport_code