可以在一個單獨的表格列上使用多個引用？

Question

給出的以下數據庫結構：

產品：

- id 
- model 
- manufacturerID 
- subcontractorID 

manufacturerID和subcontractorID被引用到同一個表，因爲有些廠商也在分包商：

製造商：

- id 
- name 

現在，如果我想選擇這兩列，我該怎麼做？

我想這樣的事情...

SELECT p.model, m.name AS Manufacturer, m.name AS Subcontractor 
FROM Product AS p 
LEFT JOIN Manufacturers AS m ON p.manufacturerID = m.id 
LEFT JOIN m ON p.subcontractorID = m.id 

...但我只能得到兩列值之一。

Answer 1

關閉 - 你需要加入的廠商表兩次：

SELECT 
    p.model, manufacturer.name AS Manufacturer, 
    subcontractor.name AS Subcontractor 
FROM Product AS p LEFT JOIN 
    Manufacturers AS manufacturer 
    ON p.manufacturerID = manufacturer.id LEFT JOIN 
    Manufacturers AS subcontractor 
    ON p.subcontractorID = subcontractor.id 

Answer 2

試試這個：

LEFT JOIN Manufacturers AS mman ON p.manufacturerID = mman.id 
LEFT JOIN Manufacturers AS msub ON p.subcontractorID = msub.id