题解 | #所有用户对高难度题目的刷题数#

SELECT
    device_id
    ,count(question_id)question_cnt
FROM
    user_profile LEFT JOIN (SELECT device_id,question_id
                            FROM question_practice_detail 
                                LEFT JOIN question_detail USING(question_id)
                            WHERE difficult_level='hard')question_info
    USING(device_id)
GROUP BY 
    device_id
ORDER BY 
    question_cnt;