Find the number of routes from the United States to Canada.
create table airports (dst_apid varchar, src_apid varchar, apid varchar, country varchar, PRIMARY KEY (dst_apid)); create table routes (dst_apid varchar, src_apid varchar, apid varchar, country varchar, PRIMARY KEY (dst_apid))
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')