CREATE TABLE FLIGHTS (DestAirport VARCHAR(255), SourceAirport VARCHAR(255)); CREATE TABLE AIRPORTS (AirportCode VARCHAR(255))
What is the code of airport that has the highest number of flights?
    
SQL: SELECT T1.AirportCode FROM AIRPORTS AS T1 JOIN FLIGHTS AS T2 ON T1.AirportCode = T2.DestAirport OR T1.AirportCode = T2.SourceAirport GROUP BY T1.AirportCode ORDER BY COUNT(*) DESC LIMIT 1