Re: Q: ISSUE-41 bNode semantics

On Thu, 2011-05-19 at 14:22 +0200, Alexandre Bertails wrote:
> On Thu, 2011-05-19 at 07:10 -0500, David McNeil wrote:
> > 
> >         There is a third way in the case the WG decides to explore a
> >         correct mapping with NULL values. I propose to translate a
> >         NULL value as a special constant from a special datatype, and
> >         to understand how SPARQL 1.0 queries should be modified in
> >         order to behave properly in presence of RDF data coming from a
> >         direct mapping of a RDB with NULL values. My guess is that it
> >         is enough to enrich the BGP part with a conjunct
> >         NOT-EQUAL(X,'NULL') [pardon my naive syntax here] for each
> >         joined (namely repeated in the BGP) variable X, so we remain
> >         in pure SPARQL 1.0.
> > 
> > I believe that I understand Enrico's point. I think this applies
> > mostly to ISSUE-42 (Direct Mapping & NULL values) rather than ISSUE-41
> > (R2RML & NULL values), although it might make sense to facilitate this
> > approach in R2RML.
> > 
> > One use case for the Direct Mapping is:
> > 
> > 1) Create a Direct Mapping for a relational database to produce
> > (either materialized or virtual) triples that comply with a generated
> > ontology.
> > 
> > 2) Define RDF-to-RDF transformation using rules, inferencing, etc to
> > convert these Direct Mapping triples into domain specific triples that
> > comply with a domain specific ontology.
> > 
> > In order to perform the RDF-to-RDF transformation _without_
> > referencing the original relational databases it is necessary for the
> > Direct Mapping triples to preserve all of the information from the
> > original relational databases. This implies that the NULL values from
> > the relational databases need to be represented in the Direct Mapping
> > triples (e.g. as Enrico described above). With those NULL values in
> > the Direct Mapping triples then it is possible to interpret the NULL
> > values as appropriate (in potentially schema specific ways) for the
> > target domain ontology.
> 
> I still don't understand why one can't use
> http://www.w3.org/TR/2011/WD-sparql11-query-20110512/#negation in that
> case...
> 
> Again, we have already agreed we would provide the information about the
> schema, meaning that is actually possible to retrieve the NULL values. 
> 
> Is it right?

I meant something like that:

SELECT ?s WHERE 
{
    ?s <#comesFromTable> ?someTable .
    ?someTable <#hasThePredicate> ?p .
    FILTER NOT EXISTS { ?s ?p ?notNullInThatCase }
}

Alexandre.

> 
> Alexandre.
> 
> > 
> > Furthermore with the NULL values present in the Direct Mapping triples
> > it is possible (albeit cumbersome) to write SPARQL queries that take
> > into account the SQL semantics of NULL. For example, to check for the
> > RDF NULL in the queries and handle NULLs as needed for either
> > aggregation or straight querying.
> > 
> > The NULL value needs to be included for cases like:
> > 
> > ID NAME
> > 100 Joe
> > 200 Bob
> > 300 Sue
> > 
> > ID AGE
> > 100 30
> > 200 NULL
> > 
> > In this example the generated <bob> <age> <NULL> triple is different
> > than the lack of such a triple for Sue.
> > 
> > -David
> > 
> 
> 
> 
>

Received on Thursday, 19 May 2011 12:31:12 UTC