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 as tag,
    count(*) as tag_cnt
from
    exam_record er
    left join examination_info ei on er.exam_id = ei.exam_id
    inner join 
     (
        SELECT
            distinct uid
        FROM
            exam_record
        where
            submit_time is not null
        GROUP BY
            uid,
            month(submit_time)
        HAVING
            count(submit_time) > 2
    ) u on er.uid = u.uid
group by
    tag
order by
    tag_cnt desc