select tag,difficulty,
round(avg(score),1) as clip_avg_score

from examination_info ei
join exam_record er
on ei.exam_id = er.exam_id

where tag='SQL'
and difficulty ='hard'
and score not in (
    select max(score) as extreme from exam_record
union all 
select min(score) as extreme from exam_record )

group by tag,difficulty

难点在于通过子查询去除最高分和最低分