题解 | #月均完成试卷数不小于3的用户爱作答的类别#

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 t1 left join examination_info t2
    on t1.exam_id = t2.exam_id
    where submit_time is not null
    group by 1,substring(start_time,1,7)
    having count(submit_time) >= 3
)
group by 1
order by 2 desc;