with a as(
select 
uid,
month(start_time) as m,
count(submit_time) as cnt

from exam_record er
join examination_info ei
on er.exam_id = ei.exam_id

group by uid,month(start_time)
having cnt>=3)

select tag,
count(*) as tag_cnt

from  exam_record er
join examination_info ei
on er.exam_id = ei.exam_id
where uid in (select uid from a)

group by tag
order by tag_cnt desc

题目表达能力有待提高