Re: Constraints and Functional Dependencies
Date: Sat, 24 Feb 2007 19:28:09 GMT
> Cimode 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.
>> Critisicm. >> >> First: If I apply the above definition, all relations R that have the >> same values than S will be considered primary key for S.
> Obviously you did not yet read the responses to the OP when you wrote
> this. By now you may have. This is the same point paul c raises in his
> second reply to the OP, stated another way, right?
> One minor nitpick: The distinction between *primary* key and other keys
> is considered to be a practical choice for implementing DBMS's.
> As long as there is no established theoretical
> need for this distinction, we can just use these:
> key and candidate key (candidate key if there are more keys).
> Slightly more important: at this stage in the OP's
> argumentation the term (candidate) key has not yet been defined.
It looked to me as if the OP did define it. I've seen the same definition expressed in equivalent algebra here many moons ago.
p Received on Sat Feb 24 2007 - 20:28:09 CET