Mysql：从多个表中获取数据

Question

我有 4 张桌子。

帖子：id、标题

标签：id、名称

post_tag：id、post_id、tag_id

user_read_activity：id、user_id、post_id

User_read_activity 包含用户已阅读的帖子。 Post_tag 包含链接到标签的帖子。一篇文章可以有多个标签。我需要根据用户关注的某些标签来获取用户未读的帖子，并删除用户可能不喜欢的某些帖子。

我想出了这段代码。但这是错误的。这也给了我用户阅读的帖子。但基于标签，这些是正确的。只需要更正未读的部分。请帮我解决这个问题。

如果代码有任何错误，也请帮助我更正代码。

SELECT DISTINCT post.* FROM post 

INNER JOIN post_tag ON post.id = post_tag.post_id 

INNER JOIN tag 

WHERE tag.id = 21 OR tag.id = 26 OR tag.id = 63 OR tag.id = 86 OR tag.id = 11 

AND post.id != 1088 AND post.id != 338 AND post.id != 1396 

AND post.id NOT IN (SELECT post_id from user_read_activity WHERE user_id = 70)  

ORDER BY post.likes DESC LIMIT 5

Answer 1

你只需要在你的条件周围加上括号来明确优先顺序。您还应该在 post_tag 和 tag 之间进行适当的连接：

SELECT DISTINCT post.* FROM post 

INNER JOIN post_tag ON post.id = post_tag.post_id 

INNER JOIN tag ON post_tag.tag_id = tag.id

WHERE (tag.id = 21 OR tag.id = 26 OR tag.id = 63 OR tag.id = 86 OR tag.id = 11)

AND post.id != 1088 AND post.id != 338 AND post.id != 1396 

AND post.id NOT IN (SELECT post_id from user_read_activity WHERE user_id = 70) 

ORDER BY post.likes DESC LIMIT 5

或者@dimwittedanimal 建议的IN 标准也可以。

SELECT DISTINCT post.* FROM post 

INNER JOIN post_tag ON post.id = post_tag.post_id 

INNER JOIN tag ON post_tag.tag_id = tag.id

WHERE tag.id IN (21, 26, 63, 86, 11)

AND post.id NOT IN (1088, 338, 1396)

AND post.id NOT IN (SELECT post_id from user_read_activity WHERE user_id = 70) 

ORDER BY post.likes DESC LIMIT 5