Show the route id of the destination airport id 2990?
select rid from routes where dst_apid = '2990'
Show the number of different airports which are the destinations of the American Airlines?
select count ( distinct dst_apid ) from airlines as t1 join routes as t2 on t1.alid = t2.alid where t1.name = 'American Airlines'
Show the destination airport of route id 39?
select dst_apid from routes where rid = 39