题解 | #月均完成试卷数不小于3的用户爱作答的类别#

select ei.tag,count(er.start_time) cnt from exam_record er,examination_info ei
where er.exam_id=ei.exam_id
and er.uid in(
select a.uid from exam_record a
where a.submit_time is not null
group by a.uid
having (count(a.exam_id)/count(distinct substr(a.submit_time,1,7)))>=3
)
group by ei.tag
order by cnt desc

1.先求出月均完成试卷数的uid

2.在根据uid取关联求出课程对应的课程次数

也许这个不是最优解，如果您有更好的办法，欢迎留言解答