SELECT u.university, COUNT(q.question_id) / COUNT(DISTINCT u.device_id) AS avg_answer_cnt FROM question_practice_detail AS q LEFT OUTER JOIN user_profile AS u ON u.device_id = q.device_id GROUP BY u.university; 区分内联结（INNER JOIN）与左外联结（LEFT OUTER JOIN）、右外联结（RIGHT OUTER JOIN）的区别：INNER JOIN联结两个表时，只保留两个表...
