Re: object algebra

"Neo" <neo55592_at_hotmail.com> wrote in message news:4b45d3ad.0402242201.72ab2c26_at_posting.google.com...
> > If Mary has no eyes, then the definition of a person as having eyes is
> > flawed.
>
> Are you saying the following set is valid or allowed in RDM?
> Color = {Red, green, blue, "Not Applicable", "Unknown"}
>
> Is "Not Applicable" a color?

This question is really a good one and was not answered well !!

Domain should be from the problem space (semantics). When we talk about eye colors there are no N/A or UNK eye colors in the world. There are just "people" (tuples) who don't have eyes and therefore don't have eye colors, or people who have eyes but we don't know the color of them. So I thing we have to leave the domain intact.

Introducing N/A nad UNK to the domain Color (or Eye_Color) we make the same thing as when augmenting the real set R with two aditional elements (+ Infinitiy and - Infinity) and therefore introducing a new set R* ... then we lose closure in the domain R* (e.g. Inf - Inf = ???) that we had in R....

So we do not really solve the problem. That means that almost every domain I use in my relational databases should have N/A and UNK members .. then I have to redefine the algebra over those domains (e.g. myDate: today + N/A = N/A). Have we done anything better ? I don't think so.

I see no "poblem solving" with domains like that....

Greetings from Croatia,
Drago Ganic Received on Sat Feb 28 2004 - 14:37:37 CET