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Re: Does DASL Need to Support Structured Queries?

From: Slein, Judith A <JSlein@crt.xerox.com>
Date: Tue, 17 Nov 1998 15:30:54 -0500
Message-ID: <201BB34B3A73D1118C1F00805F1582E8B76CFD@x-wb-0128-nt8.wrc.xerox.com>
To: "'www-webdav-dasl@w3.org'" <www-webdav-dasl@w3.org>
I think I would assume based on the DASL charter and the requirements that
it would be possible to query on any property that could be exposed through
WebDAV.  That would certainly include structured properties, although I see
that there is no explicit requirement about structured properties.

Aside from meeting expectations, DASL would be dramatically more useful if
it could accept queries on structured properties.  The arguments already
presented on the DASL mailing list are compelling: There are already
structured properties in the DAV: namespace, and there will certainly be
more when the rest of the WebDAV specifications are in place.  Just finding
out basic information like what resources I have locked would require a
structured query.

Imagine an application that lets users construct queries.  It presents the
user with a list of queryable attributes, and a lot of the properties that
the user knows exist are not on the list.  Why not?  Because they are
structured.  I would find this extremely frustrating.  Why can't I query on
creator?  Because it's a list-valued property.  Why can't I query on
resource type? Because its values are XML elements rather than strings.

I think basically you need to be able to test whether a particular element
is present, since many structured properties have empty elements at the leaf
level; and you need to be able to compare the value of a particular element
of a structured property with a literal.

A case I'm particular concerned about at the moment is searching for
references to a given resource.  This involves testing the DAV:href element
of the DAV:reftarget property for equality with a string literal.  If you
wanted only the direct references to the given resource, you would also test
for DAV:reftype with the value DAV:direct (an empty element).

Other more esoteric searches might be interesting, but less critical.  For
example, find resources where the nth child of the author element is
"Smith"; or find resources where the xml:lang attribute of the title
property is "de"; or sort resources by documenttype, where the values of
documenttype are empty xml elements.

I like Alan's notion of using a proppath element to specify which element of
a property we want to test.  I find his syntax for testing for the existence
of an element very unintuitive.  I might rather just let the isdefined
element apply to proppath as well as to prop.


Judith A. Slein
Xerox Corporation
800 Phillips Road 105/50C
Webster, NY 14580
Received on Tuesday, 17 November 1998 15:27:16 UTC

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