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(exam_id) as pv
    from
        exam_record
    group by
        exam_id
    order by
pv desc 
) as combinedform1
union
select * from 
(
    select
        question_id as tid,
        count(distinct uid) as uv,
        count(question_id) as pv
    from
        practice_record
    group by
        question_id
    order by
pv desc
) as combinedform2

本题的是需要把question_id和exam_id的值结合到一个列表的属性中，需要通过union关键词，但是要考虑到在union组合后，子查询的排序是不会生效的，需要在子查询外再嵌套一层select，代码如上所示。