#方法1 SELECT u.university,ROUND(COUNT(q.question_id)/COUNT(DISTINCT q.device_id),4) AS avg_answer_cnt FROM user_profile u,question_practice_detail q WHERE u.device_id=q.device_id GROUP BY university; #方法2 SELECT u.university,ROUND(COUNT(q.question_id)/COUNT(distinct(q.device_id)),4) avg_answer_cnt FR...