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   Re: [xml-dev] XML Data Modellling/Linking (was RE: [xml-dev]AfterXQuery,

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On Sat, 2004-10-23 at 05:28, Michael Champion wrote:
> On Fri, 22 Oct 2004 21:15:54 GMT, rpbourret@rpbourret.com
> <rpbourret@rpbourret.com> wrote:
> > Michael Kay wrote:
> > 
> > >  What happened to the old doctrine of data independence?
> > 
> > Hmmm... Sounds like the relational model. (Sorry, couldn't resist.)

I was tempted to answer "Sounds like the RDF model" when I first read
Michael's post.

> > I think this could actually be dealt with at the query language level,
> I hope you're not joking :-)  
> The CODASYL data model ultimately foundered because of its unwieldy
> links, and XLink foundered trying to do something similar for XML. 
> Maybe the lesson here is that the relational model approach of
> defining links *dynamically* based on relationships on the *values* of
> information items rather than predefined links really is the way to do
> what XLink tried to do.
> The thing I always liked best about XQuery is simply the addition of a
> Join operation into the XML corpus.  Until this thread I hadn't
> thought of this as a replacement for XLink, but that idea is starting
> to take root in my head ... it really might be worth a look back at
> the struggle between advocates of CODASYL and those for the relational
> model to see if there is a lesson there for us.  Date's little book
> called "The Database Relational Model: A Retrospective Review and
> Analysis : A Historical Account and Assessment of E. F. Codd's
> Contribution to the Field of Database Technology" is the best summary
> of this that I know of ...and the fact that Date and friends are
> fervent opponents of XML should not  dissuade us from carefully
> considering his analysis :-)

I think that there are two equally important (and interdependent)
reasons why the relational model wiped out CODASYL.

One is, as you point out, the query language and the other is the
simplification of the model to isolate simpler atoms: tables and rows,
cells are easier to manipulate, query and recompose (as graphs or trees)
than tree fragments.

To do to XML what the relational model has done to CODASYL, I think that
we need not only a query language but also to break tree fragments into
atoms that are easier to manipulate, query and recompose and RDF strikes
as the candidate that seems (at least technically) able to do so today
(by RDF, I mean the RDF triples basic data model, not the XML syntax).

In other words, IMO the join operation isn't enough if all you can join
are tree fragments that are by nature not "merge friendly" and one of
the biggest (and usually underestimated) benefit of RDF is its ability
to "auto-merge" information from multiple sources. 

Read me on XML.com.
Eric van der Vlist       http://xmlfr.org            http://dyomedea.com
(ISO) RELAX NG   ISBN:0-596-00421-4 http://oreilly.com/catalog/relax
(W3C) XML Schema ISBN:0-596-00252-1 http://oreilly.com/catalog/xmlschema


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