题解 | 每个题目和每份试卷被作答的人数和次数

select
    *
from
    (
        select
            exam_id as tid,
            count(distinct uid) as uv,
            count(uid) as pv
        from
            exam_record
        group by
            exam_id
        order by
            uv desc,
            pv desc
    ) t1
union
select
    *
from
    (
        select
            question_id as tid,
            count(distinct uid) as uv,
            count(uid) as pv
        from
            practice_record
        where
            submit_time is not null
        group by
            question_id
        order by
            uv desc,
            pv desc
    ) t2

这个题目提供了一个思路，就是由于union本身对排序的时候，只能在union的最后使用一次order排序，因此如果想针对子查询分别进行排序的话，那么就可以进行一次查询并完成排序后，作为数据源再反给外层查询，然后再进行union