#思路：①先选出9月份完成试卷>=3的uid；②两表连接，将之前选出的uid作为子查询条件，再根据tag分组计数即可；
select tag, count(tag) tag_cnt
from exam_record left join examination_info using(exam_id)
where uid in
(select uid
from exam_record
where date_format(submit_time,'%Y%m')='202109'
group by uid 
having count(score)>=3)
group by tag
order by tag_cnt desc