如何在MySQL中选择每行的两个元素的最大值

Question

我有一个表，它是 (My)SQL 查询的结果。在此表中，我有帖子创建时间戳和用户评论创建时间戳。诀窍是并非所有帖子都有评论（所以有些 comment_creation 是 NULL）。

我想根据帖子或用户评论的最近创建时间对行进行排序。

如何获取每行的max(post_creation, comment_creation) 并对其进行排序（DESC order）？

感谢所有贡献。

Answer 1

根据您之前的问题，尝试：

SELECT p.id AS post_id, 
       p.author_id AS post_author_id, 
       p.created_date AS post_created,
       c.author_id AS comment_author_id,
       c.created_date AS comment_created,
       p.title, 
       c.content,
       coalesce(c.created_date,p.created_date) AS sort_date
FROM Posts p 
LEFT JOIN Comments c ON p.id = c.post_id
WHERE p.author_id = $userId
UNION ALL
SELECT p.id AS post_id, 
       p.author_id AS post_author_id, 
       p.created_date AS post_created,
       c.author_id AS comment_author_id,
       c.created_date AS comment_created,
       p.title, 
       c.content,
       c.created_date AS sort_date
FROM Posts p 
RIGHT JOIN Comments c ON p.id = c.post_id
WHERE c.author_id = $userId
ORDER BY sort_date

Answer 2

鉴于您的桌子看起来像这样......

CREATE TABLE `yourtable` (
  `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
  `post_creation` timestamp NULL DEFAULT NULL,
  `comment_creation` timestamp NULL DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM;

...SELECT-query 可以这样完成：

SELECT IF(comment_creation > post_creation, 
          comment_creation, 
          post_creation) AS sortorder,
       id
FROM yourtable
ORDER BY sortorder DESC;