Context: CREATE TABLE airports (dst_apid VARCHAR, src_apid VARCHAR, apid VARCHAR, country VARCHAR); CREATE TABLE routes (dst_apid VARCHAR, src_apid VARCHAR, apid VARCHAR, country VARCHAR)
Question: Find the number of routes from the United States to Canada.
Answer: 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')