Find all airlines that have flights from airport 'CVO' but not from 'APG'.
create table airlines (airline varchar, uid varchar, PRIMARY KEY (airline)); create table flights (airline varchar, sourceairport varchar, PRIMARY KEY (airline))
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"