现做后面两个表的内连接,在与第一个表做内连接,这个avg_answer_cnt感觉怪怪的 SELECT university, difficult_level, count(*)/count(distinct A.device_id) as avg_answer_cnt FROM user_profile A inner join (select device_id, difficult_level from question_practice_detail B inner join question_detail C on B.question_id = C.question_id) D o...