Question

我试图确定某位用户是否喜欢过帖子，但我收到了重复的结果...我是否遗漏了某些内容，或者是否有更好（更有效）的方法来检索此数据？

SELECT 
p.*, 
u.firstname, 
u.lastname, 
l.post_id AS liked 
FROM `posts` p 
LEFT JOIN `users` u ON u.id = p.author_id 
LEFT JOIN `likes` l ON l.user_id = p.author_id 
WHERE p.author_id=1 AND p.published = 1 
ORDER BY p.post_date DESC LIMIT 0, 10

MySQL表：

**users table**
id, firstname, lastname

**posts table**
id, author_id, post, published, post_date

**likes table**
id, post_id, user_id, timestamp

这是输出：

id, author_id, post, firstname, lastname, liked
"36","1","sdfsdf","John","Smith","36"
"36","1","sdfsdf","John","Smith","4"
"27","1","xcvxvc","John","Smith","36"
"27","1","xcvxvc","John","Smith","4"
"29","1","sdfsdf","John","Smith","36"
"29","1","sdfsdf","John","Smith","4"
"3","1",""fdgdfg","John","Smith","36"
"3","1",""fdgdf","John","Smith","4"
"28","1","data","John","Smith","36"
"28","1","data","John","Smith","4"

有什么想法吗？

Answer 1

我没有意识到你可以在JOIN上使用条件语句，所以我最终这样做是为了解决我的问题：

SELECT 
p.*, 
u.firstname, 
u.lastname, 
l.post_id AS liked 
FROM `posts` p 
LEFT JOIN `users` u ON u.id = p.author_id 
LEFT JOIN `likes` l ON l.user_id = p.author_id 
  AND l.post_id = p.id 
WHERE p.author_id=1 
AND p.published = 1 
ORDER BY p.post_date DESC LIMIT 0, 10

Answer 2

您有一个典型配置，用于定义posts和users之间的多对多关系。但是，有一种特殊的方法可以使用您没有完成的交集表likes将这些表连接在一起。

交集表包含两个外键，两个表中的每一个都相互关联。在likes的情况下，这是分别引用users表和posts表的user_id和post_id字段。要正确连接这些表，likes使用user_id字段加入users表，使用post_id字段加入posts表。

表格的顺序对于解决关系并不重要。在您的情况下，因为您希望列出所有帖子，即使没有＆＃34;喜欢＆＃34;与它们相关联，然后posts表首先出现。但是，交叉表从不会持续到最后。它必须位于两个实体表之前，或者在列表中位于它们之间。每个实体表都连接到交集表，而不是相互连接。

像这样：

SELECT  p.*, u.firstname + ' ' + u.lastname AS LikedBy 
FROM    `posts` p 
LEFT JOIN `likes` l
    ON  l.post_id = p.id 
LEFT JOIN `users` u
    ON  u.id = l.user_id 
WHERE   ...

返回带有两个左连接的重复mysql结果

2 个答案: