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 exam_record
    group by uid
    having count(submit_time)/count(distinct date_format(submit_time,"%Y%m")) >= 3
)
group by tag
order by tag_cnt desc