题解 | #月均完成试卷数不小于3的用户爱作答的类别#

select 
b.tag,
count(*) cmt
from exam_record  a 
left join examination_info b on a.exam_id=b.exam_id
inner join (
    select distinct uid -- 多次运行不通过原来是少了一个distinct 导致数据膨胀。
    from exam_record 
    where submit_time is not null
    group by uid,date_format(submit_time,'%Y%m')
    having count(submit_time)>=3
    )c on  a.uid=c.uid
group by b.tag
order by cmt desc;