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   Re: Fw: XML query language

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  • From: Paul Prescod <paul@prescod.net>
  • To: XML List <xml-dev@ic.ac.uk>
  • Date: Wed, 31 Mar 1999 11:48:59 -0600

Oren Ben-Kiki wrote:
> >You don't need an element. You just need a nodelist. Look at the DOM's
> >brutally named "getElementsByTagName" method.
> You mean the NodeList contains the matched nodes directly, and not XPointers
> which point to them. 

Right. Pointers to, not copies of, the nodes. And the pointers should be
in the most efficient "syntax" allowed by the system. In a Python program
it is a PyObject reference. In C++ it is a DOMNode *. In a
process-portable XML encoding it is an XPointer. Everybody is focused on
this last case but it is only a special case.

> >If you are asking me what is the syntax for a nodelist then I'll say it
> >has no syntax. It is an abstraction like the record set returned by a
> >database. If you have to move the query result between machines then you
> >can choose an encoding (quite likely XML) but that's outside of the realm
> >of the query language itself -- it is akin to report writing.
> No standard way to represent a query result as text? I find this strange.

I didn't say that there should be no standard way. I said that the
standard way is not something that the query language should specify. If
there are 6 query languages (some standardized and some proprietary) and 6
result encoding syntaxes (some standardized and some proprietary) then you
should be able to use any query language with any encoding syntax.

> Both XML-QL and XQL have ways to construct results (CONSTRUCT and
> <xql:result>). 

There is no such element type described in

> OK, if what you are saying is:
> - We have two languages:
>   (i) matching of XML elements, which we'll call XQL for the moment, and is
> basically the XSL match pattern language;
>   (ii) constructing XML trees from other XML trees which we'll call XTL for
> the moment and is basically the <xsl:*> tags.
> - XSL is the combination of both (plus FO objects).
> - XQL is usable in other contexts then XTL.
> - There's no other standard XML construction syntax other then XTL.
> Then we agree. 


> I'd also add:
> - We should have separate specs for XQL, XTL, and FOs. The XTL spec should
> simply reference the XQL spec. The FO spec should be independent.

Techically a good idea but I think that it is politically impossible to
separate XSL and its matching language at this point. Maybe XSL 2.0 will
depend on whatever XML QL is eventually standardized.

> - XQL should be used wherever a set of XML elements needs to be selected
> from an XML tree.
> - So therefore CSS should allow using XQL in its selectors. For that matter,
> CSS should allow an XML syntax :-)
> - And also XPointers?

I agree with all of this but changes to CSS are unlikely in the
short->medium term.

 Paul Prescod  - ISOGEN Consulting Engineer speaking for only himself

"Perpetually obsolescing and thus losing all data and programs every 10
years (the current pattern) is no way to run an information economy or
a civilization." - Stewart Brand, founder of the Whole Earth Catalog

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