Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

From: Jon Heggland <>
Date: Fri, 10 Jun 2005 09:28:38 +0200
Message-ID: <>

In article <42a81515$>, says...
> Jon Heggland schrieb:
> > In article <42a5d881$>, says...
> >
> >>Assume you have two tables Products =<Price, Size> and Personel = <Age,
> >>Salary>. Then you have a record in Products r=<$100, big>.
> >>
> >>Question: What Age has the record r?
> >>
> >>Answer: null.
> >
> > With reservations as to the semantics of your somewhat non-intuitive
> > example tables, I prefer
> >
> > Answer: That is a nonsensical question (on several accounts).
> For me personally the question not only does make sense but it is one
> the crucial questions for understanding data semantics. But of course I
> understand that this question is not asked in RM.

If you must, you can, using the monstrosity known as outer union.

> You have dimensions in
> your database (say, 1000 in all the tables) and you have rows in this
> database. So why rows from table Products are positioned only along 2
> dimensions Price and Size?

Because those are the only ones that are relevant in your problem domain. Occam's razor?

> What if I want to get their position along other 998 dimensions?

They have none. But anyway, aren't you the one that argues that "their position is NULL" and "they have no position" is equivalent? Isn't it then mainly a matter of syntax/presentation? I just don't like to use the NULL word; it has too many meanings.

> It is not only theoretcally meaningful question -
> it is practically needed to be answered because rows can be present in
> other dimensions (have values) implicitly via other rows.

What do you mean by this? I suspect we have further trouble communicating because of the issue whether "objects" have identity beyond their properties/attributes.

> Such a model then has a canonical (primitive) semantics and we can ask
> such questions as "are two models semantically equivalent" or "is model
> 1 more specific than model 2", "how the model looks like at some level
> of details".

That (if true) might be useful for some purposes, but it is not compelling enough for me to discard the RM. Are you certain the RM does not also enable this?

Received on Fri Jun 10 2005 - 09:28:38 CEST

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