從關係表加入MySQL

Question

我想從兩個表中選擇一些數據，但它比我第一次想到的稍微複雜一些。我有兩張桌子（content & 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 | 
    +----+----------+----------+-----------+ 

的方式表是設置如下， 我創建我的網站的「物品」是由以下部分組成：

• 文章標題
• 文章正文
• 文章主類
• 文章子類別

我需要獲取每一篇文章，並從表content

因此，例如，它會帶回該行相關的主副類別 - >

another title, another body, category10, sub category 20

因爲在關係表源id爲2，鏈接到文章的elementId，targetId是與文章關聯的類別的ID。

解決此問題的最佳方法是什麼？我已經嘗試了幾種不同的查詢的得到這個工作，但它似乎並不可能

Answer 1

我認爲這是一個錯誤的烏爾關係表中的數據，這可能是基於鄰接表型號：

ID爲== 2記錄必須具有的sourceID場== 3：

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

之後ü可以試試這個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r elations表數據是確定的，而且中將sortOrder場是有道理的，那麼我們可以試試這個SQL查詢：

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