题解 | 统计每个学校各难度的用户平均刷题数
select university,difficult_level,round(count(qpd.question_id)/count(distinct qpd.device_id),4) avg_answer_cnt from question_practice_detail as qpd left join user_profile as up on up.device_id=qpd.device_id left join question_detail as qd on qpd.question_id=qd.question_id group by university,difficult_level
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