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Re: How to query items of a Sequence ?

From: Paul Gearon <gearon@ieee.org>
Date: Thu, 5 Nov 2009 15:00:59 -0500
Message-ID: <a25ac1f0911051200w3d85420bg28d31ea4f5b7b98d@mail.gmail.com>
To: Etienne Morency <etienne.morency@gmail.com>
Cc: OWL Dev <public-owl-dev@w3.org>
Hi Etienne,

This is an RDF and SPARQL issue, and not really appropriate for an OWL
list. This is especially so, given that most of the OWL community are
against the use of Containers (especially the DL folks).

I recommend mailing lists such as semantic-web@w3.org, or possibly
www-rdf-interest@w3.org.

But since I'm here....

On Tue, Nov 3, 2009 at 4:48 PM, Etienne Morency
<etienne.morency@gmail.com> wrote:
> Hi everyone,
>
> We are using sequences in our project and we are having difficulties finding
> a way to query our ontology to get all items of a sequence using SPARQL.

I mentioned that containers (including sequences) are not encouraged
by the OWL community. While I expect to see them used in general RDF,
I'm a little surprised to see them in a system described by OWL. I'm
sure there are a few people here willing to discuss this design
decision with you, but I'll just press ahead with your question. After
all, querying members of collections is just as awkward with SPARQL
1.0.  :-)

> The sequence in our ontology looks like this :
>
> <rdf:Seq rdf:about="http://www.url.com/someonto.owl#1317"/>
>
> <rdf:Seq rdf:about="http://www.url.com/someonto.owl#1317"/>
>     <rdf:_1 rdf:resource="http://www.url.com/someonto.owl#314"/>
> </rdf:Description>
>
> <rdf:Seq rdf:about="http://www.url.com/someonto.owl#1317"/>
>     <rdf:_2 rdf:resource="http://www.url.com/someonto.owl#194"/>
> </rdf:Description>
>
> <rdf:Seq rdf:about="http://www.url.com/someonto.owl#1317"/>
>     <rdf:_3 rdf:resource="http://www.url.com/someonto.owl#GENERIC_113"/>
> </rdf:Description>
>
> We are able to find the sequence "http://www.url.com/someonto.owl#1317" with
> the following query :
> SELECT ?sequence WHERE { ?sequence <" + RDF.TYPE + "> <" + RDF.SEQ + "> }
>
> But is there a way to have a query that will return the items _1, _2 and _3
> directly ?

First off, since you have a sequence, you'd often want to know what
the first thing is, the second thing is, etc, and you can select these
out just by referring to the predicates rdf:_1, rdf:_2, and so on. But
it looks like you want to get everything, so directly mentioning the
predicates won't help.

If you have RDFS entailments available, you can ask for the
rdfs:member property, since all rdf:_n are subproperties of this:

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT ?sequence ?element
WHERE { ?sequence rdf:type rdf:Seq . ?sequence rdfs:member ?element }

However, this loses your ordering. You can try to get the associated
predicate (add {?sequence ?pred ?element} and remove the rdfs:member
elements via OPTIONAL/!BOUND), but the SPARQL gets messy and we
haven't even established if you have entailment available.

If you don't have an entailment regime, then you can also try to
filter by the structure of the predicate, but you need function
extensions if your triplestore to handle that. For instance, on many
triplestores you could do:

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
SELECT ?sequence ?predicate ?element
WHERE {
  ?sequence rdf:type rdf:Seq .
  ?sequence ?predicate ?element .
  FILTER fn:starts-with( str(?predicate),
"http://www.w3.org/1999/02/22-rdf-syntax-ns#_")
}

The problem with this approach is that not every SPARQL endpoint
handles this function (yet), or they have similar though differently
named functions. Also, doing these string comparisons could get
expensive if you had a lot of data to return.

Regards,
Paul Gearon
Received on Thursday, 5 November 2009 20:01:39 GMT

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