Table schema:
CREATE TABLE FLIGHTS (DestAirport VARCHAR, SourceAirport VARCHAR); CREATE TABLE AIRPORTS (AirportCode VARCHAR)

Question: 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