Query Languages for XML

[Richard Light]

In message <346C90F2.A4566219 at isogen.com>, "W. Eliot Kimber"
<eliot at isogen.com> writes
>
>From another point of view, it's not possible to have *an* XML query
>language because there are too many different ways that you might want
>to access XML data: as nodes in groves ala SDQL, as full text using some
>full-text index, as semantic-specific objects using some domain-specific
>query mechanism, etc.
>
>So my question is: is what is desired only a new *syntax* or is there a
>requirement for a fundamentally different query mechanism?  Or have I
>entirely missed the point of the original question?

One important thing about "Standard Query Language" is that it doesn't
just query.  It is actually a complete language for "defining, accessing
and otherwise managing relational databases".

I think that anyone coming from an SQL background would find SDQL very
restricted, _in the sense that_ it provides a set of 'read-only'
functions that you can carry out on SGML documents which are, magically,
already there.  Unlike SQL, SDQL provides you with no means to:

- create a schema;
- create a new document;
- edit an existing document;
- delete a document;
- manage access to documents;
- etc.

If you want to use SDQL as the basis for document management, you would
have a very hard time of it.  And yet, surely that is what someone
looking to create and manage XML repositories is going to be interested
in having?

Obviously this is not just an XML problem: it applies equally to SGML,
which is in effect a "read only" standard.  One example of this is in
the style language's (DSSSL or XSL) online support.  As far as I am
aware, there is no support for any features of forms.  Yet, if SDQL had
primitives such as (insert-node), (replace-node) and (add-text-to-node)
it wouldn't be too hard to add an "input-line" flow object type.  And
the implications of even that simple addition would be pretty far-
reaching.

Richard.

Richard Light
SGML/XML and Museum Information Consultancy
richard at light.demon.co.uk

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