mysql union all与子查询结合效率对比

SELECT * FROM baidu_day_2014_1  WHERE account = 'xxx'
UNION ALL
SELECT * FROM baidu_day_2014_2  WHERE account = 'xxx'
UNION ALL
SELECT * FROM baidu_day_2014_3  WHERE account = 'xxx'
UNION ALL
SELECT * FROM baidu_day_2014_4  WHERE account = 'xxx'

SELECT * FROM (
SELECT * FROM baidu_day_2014_1
UNION ALL
SELECT * FROM baidu_day_2014_2
UNION ALL
SELECT * FROM baidu_day_2014_3
UNION ALL

SELECT * FROM baidu_day_2014_4) t WHERE t.account = 'xxx'

总结：第二个sql效率太低了，都下班了还没查完呢，建议第一种写法