Re: Constraints and Functional Dependencies
Date: 24 Feb 2007 10:17:19 -0800
On 24 fév, 09:01, "Marshall" <marshall.spi..._at_gmail.com> wrote:
> Questions, comments, corrections, criticisms? Anyone have any
> alternate formulas for functional dependencies or candidate
> 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.
First: If I apply the above definition, all relations R that have the same values than S will be considered primary key for S. One does not define foreign keys according to other foreign keys values but only to degree one relations.
1, A 2, A 3, B 4, C
10, A 20, A 30, B 40, C
I prefer the following formalization.
Given Relation R and relation P, the definition of R *including* a foreign key of P
x being a tuple of R,
y being a tuple of S,
for all N degree relation*s* R AND all respective empty sets of R defined as R(0): there exists one and only one 1st-degree S: for all x in R: R(x) /\ S(y) <> R(0)
Additional critiscism coming tomorrow on candidate keys and fd(on next formal expressions) Received on Sat Feb 24 2007 - 19:17:19 CET