Database: flight_2
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