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how could a foreign key ever be NULL?

From:	Terrence Brannon <metaperl(at)mac(dot)com>
To:	pgsql-sql(at)postgresql(dot)org
Subject:	how could a foreign key ever be NULL?
Date:	2001-12-17 11:38:09
Message-ID:	8BBB36A0-F2E2-11D5-92F7-003065C2A10C@mac.com
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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?

In response to

Indices at 2001-12-11 03:48:54 from sharmad

Responses

Re: how could a foreign key ever be NULL? at 2001-12-17 17:06:10 from Bruce Momjian
Re: how could a foreign key ever be NULL? at 2001-12-17 17:24:07 from Oliver Elphick
Re: how could a foreign key ever be NULL? at 2001-12-17 20:15:20 from Stephan Szabo

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