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   Re: Property role and domain (was: Playing with XML Schema & XSLT)

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  • From: ht@cogsci.ed.ac.uk (Henry S. Thompson)
  • To: Kay Michael <Michael.Kay@icl.com>
  • Date: 06 Jul 2000 13:48:35 +0100

Kay Michael <Michael.Kay@icl.com> writes:

> > "About that, I regret that the first example in the 
> > introduction to XML Schema has a tag <shippingDate> , and not
> <shipping><date>, > which is extendible, and allows machine understanding."
> >
> This need to represent both the role of a property and its domain has always
> been a problem in data modelling, for example the concept of domain was in
> Codd's original relational model but didn't make it into SQL until much
> later, and never made it into the core of the language.
> For XML I struggled a little to identify best practice on this when writing
> Chapter 4 of Wrox Professional XML: the discussion of alternative approaches
> is on page 129-130. I came to the conclusion that <Billing><Address> was
> probably preferable in theory to <Billing.Address> or <Address
> role="Billing"> or <Billing type="Address">, but rather clumsy in practice;
> and in the end I dodged making a firm recommendation.

The whole point of the separation of complex type definitions from
element declarations in XML Schema is to dissolve this difficulty.  I
am confident that the next version of XSLT will support selection on
the basis of complex type, so you can find all the elements whose
content is constrained by the 'Address' type, regardless of their

That's the point of the first example in the Primer, that two
different elements, namely <shipTo> and <billTo>, have the same type,
namely Address.

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/

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