题解 | #月均完成试卷数不小于3的用户爱作答的类别#

select tag
     , count(1) tag_cnt
from exam_record er
         inner join examination_info ei on er.exam_id = ei.exam_id
where er.uid in (select uid
                 from exam_record
                 where submit_time is not null
                 group by uid, date_format(submit_time, '%Y-%m')
                 having count(1) >= 3)
group by tag
order by tag_cnt desc ;