如何選擇MySQL中每行的兩個元素的最大值

Question

我有一個表（My）SQL查詢的結果。在這張表中，我有創建時間戳和用戶評論創建時間戳。訣竅是，並非所有的帖子都有評論（所以一些是NULL）。

我想根據帖子或用戶評論的最近創建時間排序。

我如何獲得每行的max(post_creation, comment_creation)並訂購它們（訂購DESC）？

感謝您的貢獻。

Answer 1

根據你前面的問題，請嘗試：

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

鑑於你的表看起來像這樣...

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; 

...的SELECT -query可以做這樣的：

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