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(start_time) tag_cnt	# 计算作答次数
from exam_record er inner join examination_info ei 
	on er.exam_id = ei.exam_id 
where uid in (select uid	# 子查询确定uid 在“当月均完成试卷数”不小于3的用户们
	from exam_record
	where date_format(start_time,'%Y-%m') = '2021-09'
	group by uid
	having count(submit_time) >= 3)	
group by tag
order by tag_cnt desc;