错误原因： 好几次都没有看清楚表的结构，导致浪费大量时间找出错误 select university, t3.difficult_level, count(t2.question_id)/count(distinct t2.device_id) as avg_answer_cnt from user_profile t1, question_practice_detail t2, question_detail t3 where t1.device_id = t2.device_id and t2.question_id = t...
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