CREATE TABLE routes (rid VARCHAR(255), dst_apid VARCHAR(255), src_apid VARCHAR(255), apid VARCHAR(255), country VARCHAR(255)); CREATE TABLE airports (rid VARCHAR(255), dst_apid VARCHAR(255), src_apid VARCHAR(255), apid VARCHAR(255), country VARCHAR(255))
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')