关于多对多关系的单一SQL查询

Question

我有一个简单的数据库,有几个表(和一些示例列):

帖子(ID,标题,内容)

类别(ID,标题)

PostCategories(ID,ID_Post,ID_Category)

有没有办法创建单个SQL查询,它将返回分配给每个帖子的类别的帖子？

Answer 1

您可以使用GROUP_CONCAT函数

select p.*, group_concat(DISTINCT c.title ORDER BY c.title DESC SEPARATOR ', ')
from Posts p
inner join PostCategories pc on p.ID = pc.ID_Post
inner join Categories c on pc.ID_Category = c.ID
group by p.id, p.title, p.content

Answer 2

简单的连接工作得很好.

SELECT posts.id, posts.title, categories.id, categories.title
FROM posts
JOIN posts_categories ON posts.id = posts_categories.post_id
JOIN categories ON posts_categories.category_id = categories.id