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RE: Some RDQL questions

From: Alexander Jerusalem <ajeru@vknn.org>
Date: Thu, 15 May 2003 20:02:39 +0100
Message-Id: <5.2.0.9.0.20030515190452.01a5cb10@pop.gmx.net>
To: "Seaborne, Andy" <Andy_Seaborne@hplb.hpl.hp.com>
Cc: "'www-rdf-interest@w3.org'" <www-rdf-interest@w3.org>

Thanks a lot for your reply!

> > * Is there any way to specify ordering like with the SQL order by clause?
> > * Am I right to assume that there is no support for aggregate functions?
>
>You are right - RDQL does not have the features to sort or process the
>values returned from a query.  In Jnea, they are streamed back in the order
>found and this may vary.  As RDF does not constrain the data, results can be
>a mix of plain string, resources or datatyped literals.

My problem with this is that if the database backend doesn't handle 
sorting, grouping and aggregating, I have to fetch the whole result set 
from the database process and then do it without access to indexes. That's 
a problem with large datasets.

> >* Would it be possible to query for all resources that do not have a
>certain property?
>
>Not really.  RDF does not express negation and the triple patterns matched
>on the graph also do not allow tests for the absence of something.

I'm was asking because there's the complementOf property in OWL and I 
wonder how I can implement it without this kind of negation.


> >* If I think of a TriplePatternClause in terms of SQL joins, does it
> > have inner or outer join semantics? For example if I say:
> >
> >
> > SELECT ?lastname, ?email
> > WHERE
> >          (?r, <my:lastname>, ?lastname) ,
> >          (?r, <my:email>, ?email)
>
>Both property values must exist - it is a graph pattern to match against the
>RDF graph.

So I guess I would need multiple graphs ORed together to get what I want.


> > * Does RDQL mandate anything with respect to inference along
> > subPropertyOf/subClassOf lines or is this considered an implementation
>detail?
>
>The assumption is that inference happens in the triple interfgace to the
>data being stored.  It is not a feature of the query language.

That sounds like a very elegant but hard to implement idea. Gives me 
something to think about :-)

>RDQL just looks a bit like SQL - it isn't SQL.  It is more about the
>handling of the RDF than about handling after the values have been extracted
>from the model.

Maybe I'm just abusing these technologies when I think of RDF as a flexible 
database format, of OWL as a data modelling language and of RDQL as a data 
query language. RDQL seems to suggest an in memory/in process view.

-Alexander 
Received on Thursday, 15 May 2003 15:04:00 GMT

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