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- From: Andrew McNaughton <email@example.com>
- To: Leigh Dodds <firstname.lastname@example.org>
- Date: Tue, 23 Mar 1999 06:26:54 +1200
> > we recently had the idea to use XML to express SQL-like queries
> > (so this is
> > not about querying XML -- it is about using XML to express queries). It
> > seems to me that we might not be the first ones; so has anybody defined an
> > XML document type for expressing SQL queries?
> And just to widen this question slightly - assuming I do have an XML
> of a language construct - whats the best way to do the conversion from
> the XML representation to the 'correct' language representation.
> Could I use XSL to do this - or would this be going against the grain?
> (Just to qualify this I'm relatively new to XML, and *extremely* new to
XSL doesn't seem to do very well where the desired output is not well formed.
If your SQL queries have '"', '<', '>' or '&' in them, then you're going to
start getting into kludges. perl or DSSSL would be better suited to the task.
*Why* do you want to put your queries into XML? Do you need access to the
structure of your queries? Perhaps you just need something that can be
embedded comfortably in your XML documents. What you are trying to achieve is
likely to affect how you approach the problem.
I've got a problem to tackle soon which provides an example of a reason one
might want to have queries in an XML format, and the implications it has for
encoding of my queries. It may be that others are doing similar stuff - if so
I'd like to hear about it.
I have a steady flow of news material coming through my site. I have
subscribers who receive material filtered from this according to custom
preferences. Whenever a story comes through I need my system to turn around
several thousand queries within a few minutes at worst (while not unduly
slowing my web server). I want to offer more flexible customization than I
have at present.
Basically what I need to do is to invert the problem and define a query based
on the story data which can be applied to the stored queries to find the set
of queries which the story matches. (Did that make sense?)
XML expression of queries appeals since it facilitates interchanging of
queries and data. The XML query languages I'm aware of don't seem helpful
though, as they tend to store query expressions as CDATA and don't expose the
The sort of queries I want to do are boolean logic queries. Primitives I need
are literal specification of element content or attribute content, or
containment of particular words within the element contents. Extensions of
this boolean model might include stemming (reasonably likely) and use of term
weighting (probably not). These are amply discussed in the Information
Retrieval literature for those who don't know about them.
I figure any boolean query can be expressed as a decision tree terminating in
true or false leaf nodes, that this maps well into XML, and that it should be
able to be used to search for queries matching a given document using existing
tools (eg sgrep). I believe this could lead to a relatively simple processing
model, but it remains to be seen how efficient it will be.
If anyone is aware of any relevant work that is being or has been done I'd
appreciate hearing about it. XML or otherwise.
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