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) uv, count(exam_id) pv
from exam_record group by tid order by uv desc, pv desc)t1
union all
select * from (select question_id as tid, count(distinct uid) uv, count(submit_time) pv
from practice_record group by tid order by uv desc, pv desc)t2;

order by在union中不起作用，必须放到子句中去