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Home -> Community -> Usenet -> comp.databases.theory -> Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]

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: Fri, 01 Jul 2005 22:13:11 GMT
Message-ID: <Xdjxe.135078$8T6.7279748@phobos.telenet-ops.be>


Jon Heggland wrote:
> In article <0dBwe.133168$ut1.7226423_at_phobos.telenet-ops.be>,
> jan.hidders_at_REMOVETHIS.pandora.be says...
>

>>>Yes. I know how to draw lines on a paper, but how do you do it on the 
>>>logical level (if there is such a thing)? But it seems you answer that 
>>>further down.
>>
>>Ok, but just to be sure let me answer it a bit more. It is quite easy to 
>>come up with a formal description of the syntax of such a data model, 
>>and it is also very easy to formally describe what drawing such edges 
>>exactly means, including any constraint that might be included in the 
>>notation.

>
> Do you have any examples apart from ORM? (What are the operators of ORM,
> by the way?)

Sure. FDM, IFO, LDM, HERM and even for subsets of UML class diagrams there are formalizations.

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. :-) Really, why do you think coming up with such a language would be a problem?

>>>OMR/NIAM is very nice indeed, but it is little more than a graphical 
>>>notation for the relational model. 
>>
>>I completely disagree with you here. This mapping can be far from 
>>trivial with ORM, and usually there is more than one way to map them and 
>>choosing one involves what I would describe as 'implementation decisions'.

>
> I can't claim much experience using ORM; I have only read a few books
> and articles. But my impression is that ORM's concept pretty much map
> 1:1 to the RM. Object type = domain. Reference scheme = possrep.
> Relationship type = relvar. Uniqueness constraint = key. If we renamed
> the RM terms to match, would it then be an ER model?

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.

>>>Is it really common practice to refer 
>>>to it as "ER-like"? That is doing it a great disservice, in my mind.
>>
>>You are the one who just called it "little more than a graphical 
>>notation for the RM". :-)

>
> I meant that as praise, not criticism. :)

Of course you did. :-)

> But my point was that ORM and
> Chen's ER model are not that alike. What defines whether a conceptual
> modelling notation is ER-like or not? The use of the words "entity" and
> "relationship"?

Yes, these should be the fundamental concepts and they should be used in more or less the same way.

> Are UML class diagrams ER models?

Yes.

> Are semantic networks?

Yes.

> Are XML schemas?

No.

> My main beef with ER is the relatively arbitrary separation of entities
> and relationships. ORM avoids that (though it does separate "non-lexical
> objects" / "entities" and "lexical objects" / "values", perhaps a bit
> needlessly).
>
> 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.

>>>>Formalizing the ER model is a no-brainer and that makes it a 
>>>>data model that you can compare with the RM. 

>
> [snip]
>
>>Moreover, giving a full 
>>industrial-strength formalization of the RM is actually not that easy

>
> 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?

>>have taught a few in the past) and often even more complex than doing 
>>the same directly for some ER-like data models. So given that we can 
>>give an elegant and simple formalization directly, it doesn't make much 
>>sense to give an alternative that is indirect in nature and therefore 
>>harder to understand and reason about.

>
> The RM is harder to reason about?

No, what I said is that the whole that consists of the formalization of the relational model plus the mapping of ER diagrams to that formalism, is harder to reason about.

Received on Fri Jul 01 2005 - 17:13:11 CDT

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