题解 | #统计复旦用户8月练题情况#简单理解

 SELECT
    up.device_id,
    up.university,
    count( question_id ) AS question_cnt,
    ( SELECT COUNT(*) FROM question_practice_detail WHERE result = "right" AND device_id = up.device_id  ) AS right_question_cnt
FROM
    user_profile up
    LEFT JOIN question_practice_detail qpd ON up.device_id = qpd.device_id  AND MONTH ( qpd.date ) = 8 
WHERE
    university = "复旦大学"  
GROUP BY
    up.device_id