# 使用平均值函数来计算正确率
select difficult_level, avg(result='right') correct_rate
from user_profile u
join question_practice_detail q
on u.device_id=q.device_id
join question_detail d
on q.question_id = d.question_id
where university='浙江大学'
group by difficult_level
order by correct_rate;