Question: How many routes have a source airport in Canada and a destination airport in the United States, please?
SQL: select count ( * )  from  ( SELECT rid FROM airports AS T1 JOIN routes AS T2 ON T1.apid   =   T2.dst_apid WHERE T1.country   =   'United States' intersect SELECT rid FROM airports AS T3 JOIN routes AS T4 ON T3.apid   =   T4.src_apid WHERE T3.country   =   'Canada' )