在MySQL和MariaDB中使用GROUP和ORDER

Question

我有一个表：voj_submission，它的结构如下：

CREATE TABLE IF NOT EXISTS `voj_submissions` (
    `submission_id` bigint(20) NOT NULL,
    `problem_id` bigint(20) NOT NULL,
    `uid` bigint(20) NOT NULL,
    `submission_judge_result` varchar(8) DEFAULT NULL,
) ENGINE=InnoDB AUTO_INCREMENT=1004 DEFAULT CHARSET=utf8;

表中的数据可能如下：

SubmissionID ProblemID UserID JudgeResult
1000         1000      User1   AC
1001         1000      User1   WA
1002         1000      User2   CE
1003         1001      User1   AC

下面的SQL希望得到问题的最新判断结果：

SELECT submission_id, problem_id, submission_judge_result 
FROM (
    SELECT * FROM 
    voj_submissions 
    ORDER BY submission_id DESC 
) s 
WHERE uid = 'User1'
AND problem_id >= 1000
AND problem_id < 1010
GROUP BY problem_id

在MySQL 5.6中，它正常工作，并返回以下结果集：

SubmissionID ProblemID JudgeResult
1001         1000      WA
1003         1001      AC

但是在MariaDB 10.0.14中，它返回：

SubmissionID ProblemID JudgeResult
1000         1000      AC
1003         1001      AC

这意味着ORDER BY DESC没有在MariaDB中执行。我该如何解决这个问题？

参考资料：

• https://mariadb.com/kb/en/mariadb/group-by-trick-has-been-optimized-away/
• MySQL order by before group by

Answer 1

@bluefeet在这个post中说过

在子查询中使用ORDER不是解决此问题的最佳方法。 作者获取最大值(Post_date)的最佳解决方案是使用子查询返回最大日期，然后将其连接到post_author和最大值日期的表中。

因此，解决办法应该是：

SELECT submission_id, s1.problem_id, submission_judge_result
FROM voj_submissions  s1
INNER JOIN (
    SELECT MAX(submission_id) AS max_submission_id, problem_id
    FROM voj_submissions
    WHERE uid = 1000
    AND problem_id >= 1000
    AND problem_id < 1010
    GROUP BY problem_id
) s2 
ON s1.problem_id = s2.problem_id
AND s1.submission_id = s2.max_submission_id

这在MariaDB和MySQL中都是有效的。