Re: Constraints and Functional Dependencies
Date: Sat, 24 Feb 2007 17:13:40 +0100
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.
b should be a (candidate) key of S, but that
b should be a (candidate) key of S, but thatis not yet defined at this stage. However, because the fk notion was not used to define candidate key further on, no real harm is done to the argumentation - just think of this piece of the OP as below the candidate key definition. Received on Sat Feb 24 2007 - 17:13:40 CET