select difficult_level ,round(sum(case when result = 'right' THEN 1 ELSE 0 END)/count(result),4) AS correct_rate from 
user_profile AS user join question_practice_detail AS que on user.university='浙江大学' and user.device_id=que.device_id
join  question_detail as qlevel on qlevel.question_id=que.question_id
group by difficult_level
order by correct_rate

这道困难题名不副实，只要掌握了基本语法和一定数学能力就没有问题