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Re: Simple example of need for negation operator

From: Adrian Walker <adriandwalker@gmail.com>
Date: Tue, 11 Dec 2007 09:12:04 -0500
Message-ID: <1e89d6a40712110612x5420f1eata2797f768de8cc7c@mail.gmail.com>
To: "Lee Feigenbaum" <lee@thefigtrees.net>
Cc: public-rdf-dawg-comments@w3.org
Lee --

Is there perhaps some similar SPARQL workaround to get aggregation?

That would be: count rows, count symbols, sum, max, min, and avg.

                                  Thanks,  -- Adrian

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Adrian Walker
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On 12/11/07, Lee Feigenbaum <lee@thefigtrees.net> wrote:
>
>
> Bob MacGregor wrote:
> > Once upon a time, there was speculation that the the combination of
> > SPARQL's OPTIONAL and UNBOUND
> > operators are adequate to emulate a negation-as-failure operator.
> > Previously, we produced examples
> > that contradicted that claim.   Here is another such example, which is
> > relatively simple, of a query that
> > (I believe) one cannot express in SPARQL.  It comes out of a real
> > application we are implementing for a client, i.e.,
> > its a use case.
> >
> > The application involves attaching tags to resources that specify the
> > time of first use.  In other words,
> > until the time specified has passed, the resource should not be
> > retrievable.  In the application, updates
> > are distributed across triple stores, and the tags serve to insure that
> > newly updated resources will be visible
> > at the same time across the distributed stores.  It can happen that a
> > resource is tagged more
> > than once (since it may be updated more than once).  Below is a query
> > that retrieves only Articles
> > such that none of their time tags has a newer time than the present
> > (phrased in a SPARQL-like
> > syntax):
> >
> > SELECT ?a
> > FROM model
> > WHERE
> >      (?a rdf:type ft:Article) AND
> >      UNSAID ((?a ex:timetag ?time) AND
> >                    (?time > $NOW))
> >
> > This query can also be phrased without difficulty in SQL.  If anyone
> > knows how to
> > express this in SPARQL, I'd like to see the solution.
>
> Hi Bob,
>
> In SPARQL, this query is (unedited, untested):
>
> SELECT ?a
> FROM :model
> WHERE {
>    ?a rdf:type ft:article ; ex:timetag ?maxtime .
>    OPTIONAL { ?a ex:timetag ?othertime . FILTER(?othertime > ?maxtime) }
>    FILTER (!bound(?othertime) && ?maxtime > '...now...')
> }
>
> Negation / universal quantification is not pretty in SPARQL, but it can
> usually be done.
>
> I'm CC'ing public-sparql-dev@w3.org, which is a more appropriate list
> for 'how-to' questions about SPARQL.
>
> thanks,
> Lee
>
>
Received on Tuesday, 11 December 2007 14:16:32 GMT

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