เที่ยวบินไปบัลติมอร์ไม่จำเป็นต้องใช้ตารางทั้งหมด
flights to BALTIMOREIt 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 flight.flight_id FROM airport_service, city, flight WHERE city.city_code = airport_service.city_code AND city.city_name = 'BALTIMORE' AND flight.to_airport = airport_service.airport_code