Question

我有一个表是（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来排序。

如何获取每一行的max(post_creation, comment_creation)并订购它们（DESC订单）？

感谢您的所有贡献。

Answer 1

根据您之前的问题，请尝试：

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

鉴于你的表看起来像这样......

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 - 查询可以这样完成：

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

如何在MySQL中选择每行的两个元素的最大值

2 个答案: