题解 | #月均完成试卷数不小于3的用户爱作答的类别#

1. main 通过UID和月份分组，然后临时表得到 一个 distinct UID 非常关键


	select EI.tag,
       count(ER.start_time) tag_cnt 
  from 
(select distinct ER.uid 
  from exam_record ER 
  join examination_info EI using(exam_id) 
  group by ER.uid,month(ER.start_time) 
  having count(submit_time)  >= 3 
  ) main 
  join exam_record ER using(uid)
  join examination_info EI using(exam_id)  
  group by tag 
  order by tag_cnt desc ;