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Re: RDFQ - RDF Queries in RDF

From: Eric Prud'hommeaux <eric@w3.org>
Date: Wed, 7 Apr 2004 08:56:51 -0400
To: Patrick Stickler <patrick.stickler@nokia.com>
Cc: RDF Data Access Working Group <public-rdf-dawg@w3.org>
Message-ID: <20040407125651.GD9321@w3.org>

On Wed, Apr 07, 2004 at 01:44:06PM +0300, Patrick Stickler wrote:
> 
> On Apr 07, 2004, at 12:31, ext Eric Prud'hommeaux wrote:
> >>
> >>   disjunction
> >
> >Not sure about this. Comments in line below:
> >
> >>...
> >>1. Disjunction is supported. Multiple queries in the same input
> >>graph are evaluated as disjunct, and multiple target templates
> >>in the same query are evaluated as disjunct. Thus
> >>
> >>{
> >>   [a rdfq:Query; ...]
> >>   [a rdfq:Query; ...]
> >>   [a rdfq:Query; ...]
> >>}
> >>
> >>equates to
> >>
> >>{
> >>   [a rdfq:Query; ...]
> >>OR
> >>   [a rdfq:Query; ...]
> >>OR
> >>   [a rdfq:Query; ...]
> >>}
> >>
> >>and
> >>
> >>{
> >>   [a rdfq:Query;
> >>    rdfq:target [...];
> >>    rdfq:target [...];
> >>    rdfq:target [...] ]
> >>}
> >>
> >>equates to
> >>
> >>{
> >>   [a rdfq:Query;
> >>    rdfq:target [...];
> >>OR
> >>    rdfq:target [...];
> >>OR
> >>    rdfq:target [...] ]
> >>}
> >
> >If I interpret this correctly, this is more a packaging of multiple
> >queries into one protocol exchange.
> 
> That's one way to look at it. Of course, one could view any form
> of boolean expression as a means to package/compress several
> queries into one exchange ;-)
> 
> >The useful difference being if one
> >must ask a combinatorial factor of all the disjunction optoins. For
> >reference, see FatAnnotationQuery (EP-4) [1] where the query asks for
> >two properties that may be dc1.0 or 1.1. This seems like it would
> >require four queries in RDFQ.
> 
> I think the issue here is that RDFQ may not provide for quite
> as tight a compression as other forms of expression, but one
> then has to weigh how often/widely complex boolean expressions
> will be needed/used.

Can one express queries in RDFQ where an arbitrary subset of the terms
in the graph are logical disjuctions according to a popular definition
of "disjuction" [2] ? I don't mean "Can you use a query compiler to
caluculate the set of queries that will express the disjunction?"

When characterizing the expressiveness of the query languages, we must
be honest and precise or there's no point in the exercise.

> >Another litmus for disjunction is (if your language supports safe
> >negation) whether you can limit the results to not include X or Y. If
> >you can't, you must limit it from including X and then do the work
> >locally to remove Y.
> 
> I consider these to be more "flavors" of disjunction. Just because
> RDFQ does not provide disjunction in exactly the same manner or
> to the same degree as some other QL, doesn't mean it doesn't
> support it.

[1] http://lists.w3.org/Archives/Public/public-rdf-dawg/2004JanMar/0083.html
[2] http://en.wikipedia.org/wiki/Or_(logic)
-- 
-eric

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Received on Wednesday, 7 April 2004 08:57:05 GMT

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