同一张表中的多个INNER JOIN

小编典典

同一张表中的多个INNER JOIN

sql

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MetalCode

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=?

看来我做错了。请帮忙。

阅读 215

2021-03-17

共1个答案

小编典典

您应该为表指定不同的别名。您正在呼叫所有这些人。

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=?

2021-03-17