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(*) as tag_cnt
from exam_record er
join examination_info
	using (exam_id)
where exists (
	select
		uid,
		date_format(submit_time, '%Y%m'),
		count(*) as answer_cnt
	from exam_record
	join examination_info
		using (exam_id)
	where submit_time is not null
	group by uid, date_format(submit_time, '%Y%m')
	having answer_cnt >= 3 and er.uid = uid
)
group by tag
order by tag_cnt desc