SELECT c.difficult_level,SUM(IF(b.result='right',1,0))/COUNT(b.question_id) correct_rate FROM user_profile a,question_practice_detail b,question_detail c WHERE a.device_id=b.device_id AND a.university='浙江大学' AND b.question_id=c.question_id GROUP BY c.difficult_level ORDER BY correct_rate asc 不习惯left join,这样不香吗
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