Question: Find the id of routes whose source and destination airports are in the United States.
Schema: create table routes (rid varchar, dst_apid varchar, src_apid varchar, apid varchar, country varchar, PRIMARY KEY (rid)); create table airports (rid varchar, dst_apid varchar, src_apid varchar, apid varchar, country varchar, PRIMARY KEY (rid))
SQL: select rid from routes where dst_apid in (select apid from airports where country = 'united states') and src_apid in (select apid from airports where country = 'united states')