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
) as t1

union all
select *
from 
(
    select
        question_id as tid,
        count(distinct uid) as uv,
        count(uid) as pv
    from
        practice_record
    group by
        question_id
    order by
        uv desc,
        pv desc
) as t2

# 本题考察的是对合并表之前的表进行分别排序，除了上面代码使用的在子查询中使用临时表进行排序之外，还可以在合并之后按照顺序对tid的首个字符进行排序：left（tid,1），uv,pv