考虑连接，其中有个细节，当数量相同时，要对等级进行降序排序

select `level`,count(distinct uid)as level_cnt
from examination_info e
inner join 
(select i.level,r.uid,r.score,r.exam_id from user_info i right join exam_record r 
 on i.uid=r.uid)b
on e.exam_id=b.exam_id
where b.score>80 and e.tag="SQL"
group by level
order by  level_cnt desc, `level` desc