Re: Objects and Relations

From: David BL <>
Date: 17 Feb 2007 07:05:49 -0800
Message-ID: <>

On Feb 17, 12:18 am, "JOG" <> wrote:
> On Feb 16, 4:40 am, Joe Thurbon <> wrote:
> > David BL wrote:

> I am happy to put up with the definition of an entity describing a set
> of attributes/value pairs. All I object to is the concept that these
> sets are anything but arbitrary collections.
> To some people a 'book' requires an attribute stating whether it is a
> hardback or a softback. In other contexts a book might just be
> composed of its title, its content, etc. (a book published online
> perhaps). Please don't dwell on this example, it is just off the top
> of my head to show that 'entities' are artifices and vary incredibly
> from person to person and context to context. So as far as data
> management is concerned, keep 'entities' out, and let humans resolve
> such concepts outside of the logical model.

You seem to be focusing on entity as meaning an abstract type, whereas I'm tending to think of an entity as a particular thing. Why are you doing that? It is well known that classification of entities is adhoc. Fortunately In DB systems we tend to state facts about particular things far more often than sets of things.

If I were to place an actual book in front of you, you could think of hundreds of objective propositions about it. Actually the number of possible propositions you could state about the book would seem almost unlimited.

If you were given a different book, again there would be countless propositions you could state about it. Now the book may have some fundamental differences. Therefore attributes relevant to the first book may not make sense for the second book and vice versa. This makes classification of books difficult. However we both agree that the RM copes well with that because it can represent knowledge about a single book across lots of different relations. RM has no need to develop a class hierarchy in the manner of OO (or indeed E/R diagrams).

What is more fundamental - facts about a particular entity, or the entity itself? Surely the facts are secondary - at least for physical entities.

Now you might argue that the situation changes as the entities become more nebulous. For example a company like Microsoft is not particularly well defined. However, to me that just means that some facts that one might state about Microsoft will be subjective or vague. Does this fuzzyness show that entities or facts are more fundamental? I would say neither because both the entity and the facts about it are fuzzy. Received on Sat Feb 17 2007 - 16:05:49 CET

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