SELECT u.university, (COUNT(q.device_id)/COUNT(DISTINCT u.device_id)) AS avg_answer_cnt FROM user_profile u, question_practice_detail q WHERE u.device_id = q.device_id GROUP BY u.university ORDER BY u.university; 看到每个学校,那就是分组,GROUP BY。要计算平均答题数,涉及字段计算,就是用每个学校答题的数量除以每个学校的用户数这是两个表,涉及内联结,这里采用WHERE子句...