select tag,count(start_time) as tag_cnt
from exam_record t1
left join examination_info t2
on t1.exam_id = t2.exam_id
where uid in 
(
    select uid from(
    select uid,count(submit_time)/count(distinct date_format(submit_time,"%Y%m")) cnt 
    from exam_record
    group by uid
    having cnt >= 3)a
)
group by tag
order by tag_cnt desc