select qd.difficult_level,count(if(qp.result="right",1,null)) / count(qp.question_id) correct_rate from user_profile up join question_practice_detail qp on up.device_id = qp.device_id join question_detail qd on qp.question_id = qd.question_id where up.university="浙江大学" group by qd.difficult_level order by correct_rate 准确率：结果为right的题数/总结果数量