题解 | #月均完成试卷数不小于3的用户爱作答的类别#

select  
e2.tag,
count(*) cnt
from  exam_record e1  left join examination_info e2 on e1.exam_id=e2.exam_id
WHERE
e1.uid in (
select  uid from exam_record
WHERE
submit_time is not NULL
group by uid ,date_format(start_time,'%Y%m')
having count(*)>=3
) 
group by e2.tag 
order by cnt desc

其实，了解题意 就好做了，是求用户ID完成作答大于3情况下，查看符合条件用户ID，试卷类别，这些试卷类中有几次作答