先将三个表连接起来，然后采用case when 语句进行正确率计算

from(SELECT device_id,university
from user_profile) a left join 
(select p.device_id,p.result,q.difficult_level from question_practice_detail p left join  question_detail q 
on p.question_id=q.question_id) b on a.device_id=b.device_id
WHERE a.university="浙江大学" and b.difficult_level  is not null
group by b.difficult_level
order by correct_rate ASC