-- 题目不难，但是有一个易错点，即union连接的两部分单独排序的时候，只分别加 order by 是不行的，会报错，而在union后加的order by 是默认对全局排序的，所以我想到了把两个部门分别加 order by 后再分别加个括号，结果发现虽然没有报错但是括号并没有作用，偶排序并不对，然后我就懵了，后面看了别人的答案才知道 加完括号还要用select * 外嵌才能让排序生效~~~ 以下为正确代码： select * from ( select exam_id as tid,count(distinct uid) as uv,count() as pv from exam_record group by exam_id order by uv desc,pv desc) a union select * from ( select question_id as tid,count(distinct uid) as uv,count() as pv from practice_record group by question_id order by uv desc,pv desc) b;