sql - 可以在一个单独的表列上进行多个引用吗？

Question

给定以下数据库结构：

产品：

- id
- model
- manufacturerID
- subcontractorID

制造商ID 和分包商ID 是指同一张表，因为有些制造商也是分包商：

制造商：

- id
- name

现在，如果我想选择这两列，我该怎么做？

我尝试过这样的事情......

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

...但我只能得到两个列值之一。

Answer 1

关闭 - 您需要加入制造商表两次：

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

试试这个：

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