- From: Richard Cyganiak <richard@cyganiak.de>
- Date: Thu, 19 May 2011 14:08:22 +0100
- To: David McNeil <dmcneil@revelytix.com>
- Cc: Enrico Franconi <franconi@inf.unibz.it>, Ivan Herman <ivan@w3.org>, Pat Hayes <phayes@ihmc.us>, Michael Hausenblas <michael.hausenblas@deri.org>, W3C RDB2RDF <public-rdb2rdf-wg@w3.org>
On 19 May 2011, at 13:10, David McNeil wrote: > 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. Yes -- it should say ISSUE-42 really. I changed the subject accordingly. > 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. I believe that's possible already, without translating NULLs. Where do you see difficulties with that? > 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. Not sure if I understand, but isn't this taken care of by the rdf:type and ID triples that are produced for Bob, and not produced for Sue? Let's assume the tables are called P_NAME and P_AGE. Without NULLs we'd get: <P_NAME/ID=100> a <P_NAME>; <PNAME#ID> 100; <P_NAME#NAME> "Joe". <P_NAME/ID=200> a <P_NAME>; <PNAME#ID> 200; <P_NAME#NAME> "Bob". <P_NAME/ID=300> a <P_NAME>; <PNAME#ID> 300; <P_NAME#NAME> "Sue". <P_AGE/ID=100> a <P_AGE>; <PNAME#ID> 100; <P_AGE#AGE> 30. <P_AGE/ID=200> a <P_AGE>; <PNAME#ID> 200. Best, Richard
Received on Thursday, 19 May 2011 14:11:36 UTC