Foreign Key to non-primary key

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If you really want to create a foreign key to a non-primary key, it MUST be a column that has a unique constraint on it.

From Books Online:

A FOREIGN KEY constraint does not have to be linked only to a PRIMARY
KEY constraint in another table; it can also be defined to reference
the columns of a UNIQUE constraint in another table.

So in your case if you make AnotherID unique, it will be allowed. If you can’t apply a unique constraint you’re out of luck, but this really does make sense if you think about it.

Although, as has been mentioned, if you have a perfectly good primary key as a candidate key, why not use that?

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