题解 | #所有用户对高难度题目的刷题数#
所有用户对高难度题目的刷题数
https://www.nowcoder.com/practice/e8b66c069376405bb7524b217c890b23
select t1.device_id, convert(sum(if(difficult_level='hard',1,0)),signed) as question_cnt from user_profile t1 left join question_practice_detail t2 on t1.device_id =t2.device_id left join question_detail t3 on t2.question_id =t3.question_id # where t3.difficult_level ='hard' group by t1.device_id order by question_cnt asc
这题的难点在于，需要统计没有做过的用户，所以不能将difficult_level放在where过滤条件里面，需要在统计的时候进行判断。
