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: Mon, 04 Jul 2005 21:52:16 GMT
Message-ID: <kciye.137333$vL5.7346312@phobos.telenet-ops.be>

Jon Heggland wrote:
> In article <I4Cxe.135796$Bh7.7066690_at_phobos.telenet-ops.be>,
> jan.hidders_at_REMOVETHIS.pandora.be says...
>

>>>I did not ask for query languages, I asked for operators.
>>
>>Why would you need them?

>
> To quote myself: "I want to know whether the operators they define (if
> they do) are logically different from the relational operators, and if
> so, whether they are orthogonal, sound, closed and complete.

I see. At the moment I don't think there is really such an algebra for the data models we are talking about. I know there have been a few attempts, but none that I would really call *the* algebra. Whether such an algebra can be designed and what it would have to look like is actually quite an interesting research question (even if you only consider ER models only as an interesting layer on top of the RM).

>>>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 then must assume that you think the difference between ER-like and
> non-ER-like models is the distinction between LOTs and NOLOTs, since
> that by your own admission is the main (or only?) difference between ORM
> and RM.

Er, no. It is *a* difference, I'm not sure if I would call it *the* difference.

> I also note that Halpin takes great pains to distinguish ORM from ER---
> to the point of apologising for "bashing" ER.

Oh, yes, and ORM *is* better, yet if you use these techniques the models will often look the same.

>>>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.

>
> Is your point that this is not needed for formalisation of ER-like
> models, or that it is simpler to formulate them for ER-like models?

My estimate would be that it is comparable, where FDM-like models are probably a bit simpeler.

Jan Hiddesr

Received on Mon Jul 04 2005 - 16:52:16 CDT