Re: Key attributes with list values was Re: What are the differences ...KEY

Brian Selzer wrote:
> You asked for an example, so here goes.
>
> Consider the following set of propositions:
>
> Jane Harper is married.
> Jane Smith is single.
>
> And a constraint that states that single people cannot become divorced.

Your example is quite handwavey. You haven't specified what the attributes are. Is name a single attribute, or are first and last separate? What is the key? What is the constraint exactly?

> It is impossible for you to know because that
> information was not provided.

This is a general limitation on all software, that it can only operate on information that it has been provided.

Brian Selzer wrote:
> "Marshall Spight" <marshall.spight_at_gmail.com> wrote in message
> >
> > I'm not sure we're using the terms in the same way, though.
> > Do you speak Java?
> >
> > Integer i = new Integer(1);
> > Integer j = new Integer(1);
> > System.out.println(i==j); // tests for identity
> > System.out.println(i.equals(j)); // tests for equality
> >
> > Is that how you're using the terms?
> >
> > In Java, == on a reference type tests for reference equality,
> > which is to say identity. If there were no reference types,
> > as in Prolog or SQL or whatever, then there is no identity.
>
> That's the point: a non-1NF attribute is not scalar, hence the ambiguity.

I observe you did not answer my question, but instead introduced new issues. I also observe that your post reversed the attributions between my comments and yours, which I have repaired.

In any event, the above Java code illustrates equality and identity as I use the terms. Did you understand the example code?

> > Members of a set don't have identity.
>
> Yes they do. Is a roll of quarters worth 25 cents? No, and to elucidate
> upon my previous point, the attributes that would uniquely identify each
> individual quarter may not be relevant within the universe of discourse, but
> that doesn't change the fact that there are still 40 quarters.

Again, it is a general limitation of software that it can only operate on the information it has.

> > If you want a system that supports identity, you don't want to
> > be using set theory. There are plenty to choose from, and
> > they are well-supported and popular!
>
> Yes, I do want to be using set theory. The recognition of identity does not
> alter the sound theoretical foundation that the Relational Model provides.
> It strengthens it, or better yet, completes it.

We still haven't really established what you mean by "identity", although you've said it's not the same thing as keys.  

Marshall Received on Mon Feb 27 2006 - 00:45:30 CET