From: | Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us> |
---|---|
To: | Terrence Brannon <metaperl(at)mac(dot)com> |
Cc: | pgsql-sql(at)postgresql(dot)org |
Subject: | Re: how could a foreign key ever be NULL? |
Date: | 2001-12-17 17:06:10 |
Message-ID: | 200112171706.fBHH6Bh06057@candle.pha.pa.us |
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Lists: | pgsql-sql |
> 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?
--
Bruce Momjian | http://candle.pha.pa.us
pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
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