how many routes are there?
select count(*) from routes
what are their source airports? List airport names.
select t1.name from airports as t1 join routes as t2 on t1.apid = t2.src_apid
For each of those airports, how many routes does it have?
select count(*), t1.name from airports as t1 join routes as t2 on t1.apid = t2.src_apid group by t1.name
order the result by decreasing number of routes
select count(*), t1.name from airports as t1 join routes as t2 on t1.apid = t2.src_apid group by t1.name order by count(*) desc