#筛选用户
with t1 as (select uid,count(uid) from exam_record er
where date_format(start_time,'%Y-%m') = date_format(submit_time,'%Y-%m') and date_format(submit_time,'%Y-%m') = '2021-09' #原来当月指的是9月
group by uid
having count(uid) >= 3
),
#合并表
 t3 as (
select t2.exam_id,tag from examination_info ei 

 join 

 (
select t1.uid,exam_id from t1
join exam_record er
on t1.uid = er.uid
) t2

on ei.exam_id = t2.exam_id)

#结果
select tag,count(tag) as tag_cnt from t3
group by tag
order by count(tag) desc