题解 | #月均完成试卷数不小于3的用户爱作答的类别#

重点是月平均完成考试的数目大于等于三。

select t2.tag,count(t1.exam_id) tag_cnt
from  exam_record t1
left join examination_info t2
on t1.exam_id = t2.exam_id
where uid 
        in  (select uid  from exam_record
             where submit_time is not null
        group by uid having count(score)/count(DISTINCT DATE_FORMAT(start_time,"%Y%m"))>=3
         ) 
group by tag
order by tag_cnt desc