Re: Constraints and Functional Dependencies
From: mAsterdam <mAsterdam_at_vrijdag.org>
Date: Sat, 24 Feb 2007 17:13:40 +0100
Message-ID: <45e063bd$0$325$e4fe514c_at_news.xs4all.nl>
>
> I presume that if S had other attributes besides b, this definition
> would mean that b doesn't need to be a so-called primary key? (That
> would be okay with me.)
Date: Sat, 24 Feb 2007 17:13:40 +0100
Message-ID: <45e063bd$0$325$e4fe514c_at_news.xs4all.nl>
paul c wrote:
> Marshall wrote:
>> ... >> With such a system, a relation R with attribute a (which I will >> write as R(a)) having a as a foreign key into S(b) is expressed >> as follows: >> >> forall R(a): exists S(b): a = b >> >> So we can express foreign keys this way. >> ...
>
> I presume that if S had other attributes besides b, this definition
> would mean that b doesn't need to be a so-called primary key? (That
> would be okay with me.)
Not sure if I get this.
Try:
b should be a (candidate) key of S, but that