Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: Fri, 1 Jul 2005 11:50:48 +0200
In article <0dBwe.133168$ut1.7226423_at_phobos.telenet-ops.be>,
> > 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
> > 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?
> > 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. :) 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"? Are UML class diagrams ER models? Are semantic networks? Are XML schemas?
> I don't see a big fundamental difference in
> the basic concepts of ER and ORM. The biggest differences are probably
> the objectification in ORM and that you don't have explicit attributes
> but non-lexical object types. Hardly very deep issues.
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
And ORM can specify (the equivalent of) multiple candidate keys, and
keys for relationships. I really miss that in (Chen's) ER (and most
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).
What do you think are the deep issues in the difference between ORM and RM?
> >>Formalizing the ER model is a no-brainer and that makes it a
> >>data model that you can compare with the RM.
> Moreover, giving a full
> industrial-strength formalization of the RM is actually not that easy
> 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.
-- JonReceived on Fri Jul 01 2005 - 11:50:48 CEST