CREATE TABLE airlines (alid NUMBER, name CLOB, iata CLOB, icao CLOB, callsign CLOB, country CLOB, active CLOB)
CREATE TABLE airports (apid NUMBER, name CLOB, city CLOB, country CLOB, x NUMBER, y NUMBER, elevation NUMBER, iata CLOB, icao CLOB)
CREATE TABLE routes (rid NUMBER, dst_apid NUMBER, dst_ap CLOB, src_apid NUMBER, src_ap CLOB, alid NUMBER, airline CLOB, codeshare CLOB)

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