Re: how could a foreign key ever be NULL?

> The Momjian book states:
> 
> A single-column foreign key is either NULL or matches a primary key
> 
> But the problem I have with this statement is that a primary key 
> cannot be null or duplicate. And since the foreign key references 
> a value that is neither null or duplicate, how could it ever be 
> NULL?

Uh, you could have a foreign key row that doesn't match a primary key
row, and is hence NULL, can't you?

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