NW บินออกจากเมืองใดไม่จำเป็นต้องใช้ตารางทั้งหมด
what cities does NW fly out ofIt is not neccessary to use all the tables.
 city:  city_code [ varchar]  city_name [ varchar]  state_code [ varchar]  country_name [ varchar]  time_zone_code [ varchar] 
SELECT DISTINCT city.city_code FROM airport_service, city, flight WHERE city.city_code = airport_service.city_code AND flight.airline_code = 'NW' AND flight.from_airport = airport_service.airport_code