What is the code of airport that has the highest number of flights?
create table flights (destairport varchar, sourceairport varchar, PRIMARY KEY (destairport)); create table airports (airportcode varchar, PRIMARY KEY (airportcode))
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