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I have a relation between two tables. The foreign key table can have a row related in the primary table. There's a way to enforce the value of the FK column to be NULL or one of the values of the PK column in the primary table?

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Yes, why not? Did you try it?

CREATE TABLE dbo.foo(fooid INT PRIMARY KEY);

CREATE TABLE dbo.bar(barid INT PRIMARY KEY,
  fooid INT NULL FOREIGN KEY REFERENCES dbo.foo(fooid));

INSERT dbo.foo SELECT 1;
INSERT dbo.bar SELECT 1,1;
INSERT dbo.bar SELECT 2,NULL; -- succeeds
GO
INSERT dbo.bar SELECT 3,2; -- fails

If this is not what you meant, then please elaborate or re-word your question.

  • You're right. I was confused with not enforced FK. Now I understand that not enforcing the FK only can help for data migration, but otherwise a FK allways must be enforced. – Apocatastasis Apr 25 '13 at 16:14

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