同一张表中的多个INNER JOIN [英] Multiple INNER JOIN from the same table

问题描述

我有一张金属表

MetalID    integer
MetalName  text
MetalCode  text

项目表

ItemID     integer
ItemName   text
...
Metal1     int Ref.-> metals.metalID
Metal2     int Ref.-> metals.metalID
Metal3     int Ref.-> metals.metalID

我正在尝试选择三个MetalCodes

I am trying to select three MetalCodes

SELECT m.MetalCode as 'Metal1', m.MetalCode as 'Metal2',m.MetalCode as 'Metal3'
FROM Item as k
INNER JOIN Metals AS m ON m.metalID=k.metal1 
INNER JOIN Metals AS m ON m.metalID=k.metal2
INNER JOIN Metals AS m ON m.metalID=k.metal3
WHERE k.ItemID=?

看起来我做的完全错误.请帮助.

Looks like I am doing it completely wrong. Please, help.

推荐答案

您应该为表指定不同的别名.您正在呼叫所有这些人.

You should specify different aliases for your tables . you are calling all of them m.

SELECT m1.MetalCode as 'Metal1', m2.MetalCode as 'Metal2',m3.MetalCode as 'Metal3'
FROM Item as k
INNER JOIN Metals AS m1 ON m1.metalID=k.metal1 
INNER JOIN Metals AS m2 ON m2.metalID=k.metal2
INNER JOIN Metals AS m3 ON m3.metalID=k.metal3
WHERE k.ItemID=?

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