MySQL 从关系表连接

Question

我试图从两个表中选择一些数据，但它比我最初想象的要复杂一些。我有两个表(content 和 relations)。

    content
    +----+-----------+--------------------+--------------+
    | id | elementId |       title        |  field_body  |
    +----+-----------+--------------------+--------------+
    |  1 |         1 | test title         | test body    |
    |  2 |         2 | another title      | another body |
    |  3 |         3 | category 10        | null         |
    |  4 |         4 | sub category 20    | null         |
    +----+-----------+--------------------+--------------+


    relations
    +----+----------+----------+-----------+
    | id | sourceId | targetId | sortOrder |
    +----+----------+----------+-----------+
    |  1 |        2 |        3 |         1 |
    |  2 |        2 |        4 |         2 |
    +----+----------+----------+-----------+

表格的设置方式如下， 我在我的网站上创建了一篇“文章”，其中包含以下内容:

• 文章标题
• 文章正文
• 文章主类别
• 文章子类别

我需要从表内容中获取每篇文章以及关联的主类别和子类别

例如，它会带回这一行 ->

另一个标题，另一个正文，类别 10，子类别 20

因为在关系表中，源 id 是 2，它链接到文章的 elementId，而 targetId 是与文章关联的类别的 ID。

实现这一目标的最佳方法是什么？我尝试了几种不同的查询来使其工作，但似乎不可能

Answer 1

我认为您的关系表数据存在错误，这可能是基于邻接表模型的:

id == 2 的记录必须具有 sourceId 字段 == 3:

relations
    +----+----------+----------+-----------+
    | id | sourceId | targetId | sortOrder |
    +----+----------+----------+-----------+
    |  1 |        2 |        3 |         1 |
    |  2 |        3 |        4 |         2 |
    +----+----------+----------+-----------+

之后你可以尝试这个sql:

select
c.id,
c.title as article_title,
c.field_body as article_body,
c1.title as article_main_category,
c2.title as article_sub_category
from
content c
join relations as r1 on r1.sourceId = c.elementId
join content as c1 on c1.elementId = r1.targetId
join relations as r2 on r2.sourceId = c1.elementId
join content as c2 on c2.elementId = r2.targetId

如果关系表数据没问题，并且sortOrder字段有意义，那么我们可以尝试这个sql查询:

select
c.id,
c.title as article_title,
c.field_body as article_body,
c1.title as article_main_category,
c2.title as article_sub_category
from
content c
join relations as r1 on r1.sourceId = c.elementId and r1.sortOrder = 1
join content as c1 on c1.elementId = r1.targetId
join relations as r2 on r2.sourceId = c.elementId and r2.sortOrder = 2
join content as c2 on c2.elementId = r2.targetId