题解 | #月均完成试卷数不小于3的用户爱作答的类别#

# 先对月份分组，找出月完成数不小于3 的月份
# 在找出对应的用户id
# 最后通过join链接，进行计算
select tag, count(tag) as tag_cnt
from exam_record er join examination_info ei using(exam_id)
where uid in (select uid
                from exam_record 
                where month(start_time) = (select month(start_time)
                                            from exam_record
                                            group by month(start_time)
                                            having count(submit_time) >=3
                                            limit 1) 
                group by uid 
                having count(submit_time) >=3) 
group by tag
order by tag_cnt desc;