Tackling the question from the more theoretical side,
I like non-monotonic SPARQL queries like the ones modeling set difference.
E.g.
"Give me all persons *without* an email address" in a certain FOAF graph.
i) It is already folklore, that you can do that with using the !bound()
filter outside an optional, i.e.
SELECT ?X
FROM G
WHERE { ?X a foaf:Person
OPTIONAL { ?X foaf:mbox ?M}
FILTER (! bound(?X) ) }
ii) What some people might find surprising is that I can achieve the
same result without using a FILTER, more generally that I can express
SELECT ?X
FROM G
FROM NAMED <boundchecker.rdf>
WHERE
{
{ ?X a foaf:Person OPTIONAL{ ?X foaf:mbox ?M} }
GRAPH <boundchecker.rdf>{ ?M :is :unbound }
}
where <boundchecker.rdf> is the graph containing the single triple
_:b :is :unbound.
Maybe requires some thinking, but is a nice example :-)
(Short explanation: the blanknode in Graph <boundchecker.rdf> only
matches to unbound variables from the optional patttern. Note that
non-well-designed OPTIONAL patterns are not commutative, see [1].
Actually, [1] "kind of" conjectured that non-well-designed patterns are
useless, but - as this query shows - they aren't really entirely useless.)
Axel
1. http://iswc2006.semanticweb.org/items/Arenas2006bv.pdf
p.s.: Since I didn't see a similar one before, I claim copyright for
that one, basically, it is very easily generalizable to model arbitrary
queries SELECT ... P WITHOUT P'
;-)
Lee Feigenbaum wrote:
Hi everyone,
I'm putting together a "SPARQL by Example" tutorial, which is, as the
name suggests, a step-by-step introduction to SPARQL taught almost
entirely through complete, runnable SPARQL queries.
So far, I've gathered a great deal of example queries myself, but I know
that many subscribers to these lists probably have favorite queries of
their own that you might be willing to share with me.
I'm looking for:
1) SPARQL queries
2) ...that can be run by anyone (no private data sets)
3a)...either by running the query against a public SPARQL endpoint
3b)...or by using a public SPARQL endpoint that will fetch
HTTP-accessible RDF data (e.g. sparql.org or demo.openlinksw.com)
4) ...that answers a real* question
5) ...and that is fun!**
* real is in the eye of the beholder, I imagine, but I'm not looking for
"finds the predicates that relate ex:s and ex:o in this sample RDF graph"
** fun is also in the eye of the beholder. fun can be a query on fun
data; a clever query that may illustrate a particular SPARQL construct
("trick"); a query that integrates interesting information; a query with
surprising results; etc.
thanks to anyone who is able to contribute!
Lee
PS I plan to make the tutorial slides available online under an
appropriate CC license once they are completed.
--
Dr. Axel Polleres, Digital Enterprise Research Institute (DERI)
email: [EMAIL PROTECTED] url: http://www.polleres.net/
Everything is possible:
rdfs:subClassOf rdfs:subPropertyOf rdfs:Resource.
rdfs:subClassOf rdfs:subPropertyOf rdfs:subPropertyOf.
rdf:type rdfs:subPropertyOf rdfs:subClassOf.
rdfs:subClassOf rdf:type owl:SymmetricProperty.
