xml-dev - Re: Fw: XML query language

["Ingo Macherius" <macherius@darmstadt.gmd.de>]

Oren Ben-Kiki <oren@capella.co.il> wrote at 31 Mar 99, 12:12:

> Paul Prescod <paul@prescod.net> wrote:
> >I wrote:
> >> Well, them, what other way is to return a list of XPointers then to store
> >> each in an "element"?
> >
> >You don't need an element. You just need a nodelist. Look at the DOM's
> >brutally named "getElementsByTagName" method.

What a XML query should return depends on what the results are needed 
for. There is no such think as "the right way" to use an XML query 
language. Look who was on the W3C-QL workshop '98 and what they asked 
for:

1. Information Retrieval
XML seen as: Collection of text documents
Formalisms offered: Z39.50, RDF, WebSQL, PAT, ...
Query result needed: References to relevant documents

2. WWW information systems
XML seen as: Abstraction of heterogenous data sources and services
Formalisms offered: HTTP, CGI, URI
Query result: Integrated data sources and services

3. Database community (both rleational and OO):
XML seen as: Set of structured facts (order doesn't matter)
Formalisms offered: SQL, OQL
Query result: Set of (re)structured facts (order doesn't matter)

4. Document processors
XML seen as: Structured text (order matters)
Formalisms offered: XSL selectors,
Query result: Pointers to selected text fragments (order matters) for 
further processing (e.g. by XSL templates or programming languages)

5. Document transformation
XML seen as: Syntax tree
Formalisms offered: hedge automata
Query result: Transformed syntax tree

6. Hypertext community
XML seen as: Graph of structured nodes connected by Hyperlinks
Formalisms offered: XLink, XPointer
Query result: Locations within a structured node

All of those need a QL. But all have different constraints (e.g. 
Hypertext needs a QL to fit in URL) and want different results 
(pointers to documents vs. documents vs. restructured documents).

David Maier identified five fundamental operations in XML queries:
1. Selection of elements 
	depending on content, structure or attributes
2. Extraction of elements
3. Redution of elements
4. Restructuring of documents
5. Combination of elements

Looking at the user groups, e.g. neither Hypertext nor information 
retrieval will need restructuring or combination. Document processing 
will need all 5 operations. 

Right now XQL offers operations 1-3, XSL offers operations 1-4 and 
XML-QL offers operations 1-5 (with the cost of loosing order).

You suggest to use XPointers as the result of XML queries. XPointers 
from my point of view are queries by themselves. Being from the 
database community, I want restructured XML as a result. Who is right 
? No one. It just depends on the way you look at it.

	++im


--
Ingo Macherius//Dolivostrasse 15//D-64293 Darmstadt//+49-6151-869-882
GMD-IPSI German National Research Center for Information Technology
mailto:macherius@gmd.de http://www.darmstadt.gmd.de/~inim/
Information!=Knowledge!=Wisdom!=Truth!=Beauty!=Love!=Music==BEST (Zappa)

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