CREATE TABLE routes (dst_apid VARCHAR2, src_apid VARCHAR2, apid VARCHAR2, country VARCHAR2)
CREATE TABLE airports (dst_apid VARCHAR2, src_apid VARCHAR2, apid VARCHAR2, country VARCHAR2)

Find the number of routes from the United States to Canada.
SELECT COUNT(*) FROM routes WHERE dst_apid IN (SELECT apid FROM airports WHERE country = 'Canada') AND src_apid IN (SELECT apid FROM airports WHERE country = 'United States')
SELECT COUNT(*) FROM "routes" WHERE "dst_apid" IN (SELECT "apid" FROM "airports" WHERE "country" = 'Canada') AND "src_apid" IN (SELECT "apid" FROM "airports" WHERE "country" = 'United States')