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

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. :-)

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.

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.

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?

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

• Jan Hidders