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

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Sat, 02 Jul 2005 19:40:24 GMT
Message-ID: <I4Cxe.135796$Bh7.7066690@phobos.telenet-ops.be>

Jon Heggland wrote:
> In article <Xdjxe.135078$8T6.7279748_at_phobos.telenet-ops.be>,
> jan.hidders_at_REMOVETHIS.pandora.be says...
>

>>For query languages for ORM see for example 
>>http://www.orm.net/queries.html or the work by Arthur ter Hofstede et al 
>>http://citeseer.ist.psu.edu/70483.html and if you give me a week I can 
>>invent two more myself. :-)

>
> I did not ask for query languages, I asked for operators.

Why would you need them?

>>No. In ORM NOLOTs are abstract. It is more correct to say that the RM is 
>>basically ORM restricted to LOTs. A very grave and crippling restriction 
>>indeed.

>
> What does "abstract" mean?

Come on, Jon. I've taught several classes on ORM modelling and even bussiness students understood the distinction between LOTs and NOLOTs. What is that you find so difficult to understand?

> As I asked earlier: If we renamed the RM terms to match, would it then
> be an ER-like model?

The anwers is still no.

> I note that ORM does not use the term "entity", and
> that Nijssen and Halpin's original(?) book used "fact type" instead of
> "relationship type".

Different names, essentially the same concept.

>>>And ORM can specify (the equivalent of) multiple candidate keys, and 
>>>keys for relationships. I really miss that in (Chen's) ER (and most 
>>>variants thereof).
>>
>>Oh yes, and it is very easy to add all that.

>
> Of course. But if you keep adding, you generally end up with something
> different than what you started with.

The basic structure remains, only extra constraints are added.

>>>Formalising the ER model is a no-brainer, but formalising the RM is not 
>>>that easy? Is this really what you are saying?
>>
>>Of course it is. Have you ever written down a full formal definition of 
>>the relational model in set theory?

>
> Not personally, but what more do you need than definitions of value,
> domain, tuple and relation, and a minimal set of algebra operators?

The notions of database schema, database constraints, database instances and how they are exactly related.

Jan Hidders

Received on Sat Jul 02 2005 - 14:40:24 CDT