Table schema:
CREATE TABLE routes (rid VARCHAR, dst_apid VARCHAR, src_apid VARCHAR, apid VARCHAR, country VARCHAR); CREATE TABLE airports (rid VARCHAR, dst_apid VARCHAR, src_apid VARCHAR, apid VARCHAR, country VARCHAR)

Question: Find the id of routes whose source and destination airports are in the United States.
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')