MySQL连接两个表并对它们进行限制和排序

Question

有一个新闻表和一个评论表。我需要从 News 表中获取 10 篇最新的新闻文章，并加入他们并对该文章发表 1 条最新评论。

数据库结构和到目前为止我尝试过的内容在这里:

CREATE TABLE News (newsId INT AUTO_INCREMENT, text VARCHAR(1000), date DATETIME, PRIMARY KEY(newsId));
CREATE TABLE Comments (commentId INT AUTO_INCREMENT, text VARCHAR(1000), date DATE, newsId INT, PRIMARY  KEY (commentId));

INSERT INTO News (newsId, text, date) VALUES
(NULL, "1st random text", NOW()), (NULL, "2nd random text", NOW()), (NULL, "3rd NEWS text", '2017-10-10 15:30:00'),
(NULL, "4th random text", '2017-10-10 15:35:00'), (NULL, "5th random text", '2017-10-10 15:32:00'),
(NULL, "6th random text", '2017-10-10 15:34:00'), (NULL, "7th random text", '2017-10-10 15:31:00');

INSERT INTO Comments (commentId, text, date, newsId) VALUES
(NULL, "1st random comment", NOW(), 1), (NULL, "2nd random comment", '2017-10-10 15:34:00' , 1), (NULL, "3rd NEWS comment", NOW(), 2),
(NULL, "4th random comment", '2017-10-10 15:39:00', 2), (NULL, "5th random comment", '2017-10-10 23:34:00',3),
(NULL, "6th random comment", NOW(),3), (NULL, "7th random comment", NOW(),4);

SELECT N.*, C.*
FROM News N((
ORDER BY N.date DESC
LIMIT 10)
    LEFT JOIN Comments C
    ON C.newsId = N.newsId
    ORDER BY C.date DESC
    LIMIT 1);

但是这不起作用并抛出一个语法错误。不知道我怎么写这个。另外，我更喜欢没有子查询的建议，在此先感谢。

编辑。我得到的错误

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual 

that corresponds to your MySQL server version for the right syntax to use near '((
ORDER BY N.date DESC
LIMIT 10)
    LEFT JOIN Comments C
    ON C.newsId = N.n' at line 2

Answer 1

这是否解决了您的问题？

SELECT N.*, P.*
FROM News N
LEFT JOIN (SELECT c.* FROM Comments C
           LEFT JOIN Comments c1 ON(c.newsId = c1.newsId and c1.date > c.date)
           WHERE c1.date IS NULL) p
 ON (p.newsId = N.newsId)
ORDER BY N.date DESC
LIMIT 10;

您的代码存在不止 1 个问题。首先，第一个 ORDER BY 子句是错误的。 Order by 出现在查询的末尾，你不能打开括号，因为优化器认为你启动了一个子查询。

其次，“我想要每个帖子的最新评论”，这意味着您需要最新的per 组。许多 RDBMS 提供像 row_number() 这样的窗口函数来解决这个问题。不幸的是，这是 MySQL，这意味着您要么必须进行自连接(查看我的内部查询)，要么使用某种相关查询。