SELECT 
    difficult_level,
    SUM(IF(qpd.result = "right", 1,0 ))/COUNT(qpd.question_id) AS correct_rate
FROM user_profile
JOIN  question_practice_detail qpd
    USING(device_id)
JOIN  question_detail qd
    USING(question_id)
WHERE university = "浙江大学"
GROUP BY difficult_level 
ORDER BY correct_rate