Sql 来自同一个表的多个内部联接

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金属代码

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=?

---

看来我做得完全不对。请提供帮助。

您应该为表指定不同的别名。你把他们都叫做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=?

试试这个：

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

嗯，不是完全错

无论什么地方有“内部连接金属如m”，

m

都必须是独一无二的（而不是每次都是

m

）

尝试以下方法（未经测试）：

或者更简单，但每行获得一个metalcode

SELECT MetalCode
FROM Item
WHERE metalID = metal1 OR metalID = metal2 OR metalID = metal3

---

您不能有多个同名别名。您想实现什么？很好，我们在同一时间编写了完全相同的示例代码。一定是双胞胎+事实上：）还是有点不对劲。我得到的结果->锌，锌，锌，而有不同的metals@Nathan考虑到我们没有经过测试就在几秒钟内写出了这些答案，这是极有可能的。这些例子应该会让你朝着正确的方向前进。也许你的数据有问题，请检查k.metail1，k.metal2，k.metal3是否具有相同的值？是的，这才是真正的答案。几秒钟内有7个答案。

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=?

SELECT MetalCode
FROM Item
WHERE metalID = metal1 OR metalID = metal2 OR metalID = metal3