题解 | #每个题目和每份试卷被作答的人数和次数#

select exam_id AS tid
        ,count(distinct uid) AS uv
        ,count(*) AS pv
from exam_record
group by exam_id
/*order by exam_id desc
        ,count(distinct uid) desc
        ,count(*) desc
*/
union all
select question_id AS tid
        ,count(distinct uid) AS uv
        ,count(*) AS pv
from practice_record
group by question_id
order by left(tid,1) desc
        ,uv desc
        ,pv desc

分别对两个表的tid都进行排序的话，需要用left(tid,1)，提取tid左边的第一个数字，试卷是9，题目是8