求截断平均值，按照题目的要求我们直接去掉exam_id为9001中的最高值和最低值再求平均数即可。

select tag,difficulty,t.clip_avg_score 
from 
(select round(((sum(score)-max(score)-min(score))/(count(score)-2)),1) clip_avg_score,exam_id 
from exam_record 
where exam_id=9001 and score is not null) t join examination_info e 
on t.exam_id=e.exam_id;