题解 | #每个题目和每份试卷被作答的人数和次数#

#次数count(exam_id),人数count(uid) #次数count(question_id),人数count(exam_id)

select exam_id as tid,count(distinct er.uid) as uv,count(exam_id) as pv
from exam_record as er
group by exam_id
union all
select question_id as tid,count(distinct pr.uid) as uv,count(question_id) as pv
from practice_record as pr
group by question_id
order by left(tid,1) desc, uv desc,pv desc