Re: Extending my question. Was: The relational model and relational

"Bernard Peek" <bap_at_shrdlu.com> wrote in message news:oU8bYjG76rU+EwVd_at_diamond9.demon.co.uk...
> In message <b2uat1$f0t$1_at_slb9.atl.mindspring.net>, Steve Kass
> <skass_at_drew.edu> writes
>
> >Bernard,
> >
> > This isn't a matter of opinion. There is one determinant: "there are
two
> >employees named John Smith". There are many consequential
> >truths, such as "there is at least one employee", "there is an employee
> >whose first name is not Nancy", "there are at least two employees
> >whose first and last names share a common letter of the alphabet.",
> >and so on.
> >
> > I don't deny that it can be important to distinguish between two
> >John Smiths.
>
> That's not my argument. My argument is that there may be no need to
> distinguish between two real-world objects, each of which is referenced
> by a single record in a database. The relational model (and databases
> based on it) require that a distinguishing key be created even if there
> is none in the logical data structure.

A relation representing all instances of a book with a count of the instances does not distinguish between the real-world objects. The relational model requires no such key as you postulate above, which makes much of your argument moot.

> I don't dispute that there are real pragmatic reasons for accepting that
> deviation from the logical structure of the data.

As mentioned above, no deviation exists.

> But as this is a
> theory newsgroup I wanted to point out that this is a (minor) failing in
> the relational model.

With all due respect, the failing is not in the relational model.

> > I'm not redefining any words, but we have a fundamental
> >difference in understanding logical vs. physical models. You are
> >saying that the real-world scenario of books in a library must be
> >represented by a logical model that does not keep track of an
> >actual attribute of "book" (acquisition number), and then you
> >blame the model for not being able to distinguish two identical
> >books,
>
> No, that's not my objection. My objection is that the relational model
> declares that there must be a distinction, when there is no such
> requirement in the real world.

As demonstrated above and in earlier messages to Mikito, no such requirement exists. If you only need to state that you have two copies of the book on your shelf, state that directly by including an attribute recording the quantity.

> --
> Bernard Peek
> bap_at_shrdlu.com
> www.diversebooks.com: SF & Computing book reviews and more.....
>
> In search of cognoscenti
>
Received on Wed Feb 19 2003 - 04:02:42 CET