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

From: Alexandr Savinov <savinov_at_host.com>
Date: Thu, 09 Jun 2005 12:08:02 +0200
Message-ID: <42a81515$1_at_news.fhg.de>


Jon Heggland schrieb:
> In article <42a5d881$1_at_news.fhg.de>, savinov@host.com 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. 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? What if I want to get their position along other 998 dimensions? 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. To answer such questions we need to represent the model as one global multidimensional and hierarchical space where all dimensions make sense for all rows. 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".

-- 
alex
http://conceptoriented.com
Received on Thu Jun 09 2005 - 12:08:02 CEST

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