UNION 和 ORDER BY 一起使用
1.需求分组排序完 再合并
( select * from ( select a from a1 order by a2 ) as table1 ) union ( select * from ( select b from b1 order by b2 ) as table2 )
2.整体合并 再排序
select * from
(... union ....) as a 【派生表别忘了加别名,否则 Every derived table must have its own alias】
order by..
order by 别忘了在 select语句中查询出
union和union all的差别是前者会忽略重复数据