Re: Foreign keys
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Thu, 17 Jan 2008 12:27:06 -0400
Message-ID: <478f81dc$0$4062$9a566e8b_at_news.aliant.net>
>>Then the foreign key B1 represents the functional dependency
>>B1 --> A2,
>>which is the functional dependency across two relation I mentioned in
>>the first sentence.
>>
>>Furthermore, through transitivity by the functional dependency K1 -->
>>B1, the foreign key also represents the inter-relational functional
>>dependency
>>K1 --> A2.
>>
>>Am I correct to say this?
>>
>>--
>>
>>-kira
Date: Thu, 17 Jan 2008 12:27:06 -0400
Message-ID: <478f81dc$0$4062$9a566e8b_at_news.aliant.net>
Evan Keel wrote:
> "Kira Yamato" <kirakun_at_earthlink.net> wrote in message
> news:2008011502240916807-kirakun_at_earthlinknet...
>
>>On 2008-01-14 21:18:57 -0500, "Evan Keel" <evankeel_at_sbcglobal.net> said:
>>
>>
>>>Always a physical issue. Never a theory issue.Agree?
>>
>>Foreign keys are functional dependencies across two relations.
>>
>>More specifically, let
>>R1(K1, A1, B1)
>>be a relation with attribute sets K1, A1 and B1 where K1 is R1's
>>primary key and B1 is a foreign key to the relation
>>R2(K2, A2)
>>where K2 is R2's primary key and A2 is the set of its remaining
> > attributes. >
>>Then the foreign key B1 represents the functional dependency
>>B1 --> A2,
>>which is the functional dependency across two relation I mentioned in
>>the first sentence.
>>
>>Furthermore, through transitivity by the functional dependency K1 -->
>>B1, the foreign key also represents the inter-relational functional
>>dependency
>>K1 --> A2.
>>
>>Am I correct to say this?
>>
>>--
>>
>>-kira
> > Not reading the whole thread (big mistake on my part), but how about the > instance where a foreign key can be null?. No functional dependecy but > legit. > > Evan
The legitimacy is predicated on the legitimacy of NULL in the first place. Received on Thu Jan 17 2008 - 17:27:06 CET