Database: flight_2
Question: Find all airlines that have flights from airport 'CVO' but not from 'APG'.
SQL: select t1.airline from airlines as t1 join flights as t2 on t1.uid  =  t2.airline where t2.sourceairport  =  "cvo" except select t1.airline from airlines as t1 join flights as t2 on t1.uid  =  t2.airline where t2.sourceairport  =  "apg"