SELECT count(*) from (SELECT province_code as area_id, count(*) from t_cas_rural_selected GROUP BY province_code UNION all SELECT city_code as area_id, COUNT(*) from t_cas_rural_selected GROUP BY city_code UNION all SELECT county_code as area_id, COUNT(*) from t_cas_rural_selected GROUP BY county_code) as tTotal
如上，后面加个 as tablename 就可以了，例如："as tTotal"
mysql union出错: "Every derived table must have its own alias"
原文：https://www.cnblogs.com/chenmz1995/p/10263422.html