重点是理解题意

将每类试卷的最大值、最小值查出来，并根据题型、uid进行分组

这里可以进行连续排列：

partition by tag order by max_score desc,min_score desc,uid desc

（如果两人最大分数相同，选择最小分数大者，如果还相同，选择uid大者）

select  tag as tid,uid, rk1 as ranking from 
(select *, rank()over(partition by tag order by max_score desc,min_score desc,uid desc) as rk1 from 
(select tag,es.uid,max(score) as max_score,min(score) as min_score
 from examination_info ex
left join exam_record es
on ex.exam_id=es.exam_id
group by tag,es.uid ) e
)a
where rk1<=3