Re: Q: ISSUE-41 bNode semantics

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 in that

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?


> 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:
> 100 Joe
> 200 Bob
> 300 Sue
> 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 12:23:06 UTC