Re: Default Graph Contents

On Mar 9, 2011, at 19:53 , Souripriya Das wrote:

> Please see my answers inline. -- Souri.
> 
> On 3/9/2011 12:54 PM, David McNeil wrote:
>> Yesterday in the working group conference call we did not directly answer this R2RML question:
>> 
>> For a SPARQL query that does not specify any "FROM <x>" or "FROM NAMED <x>" or "GRAPH <x>" clauses, what is expected to be in the default dataset for this query? I can think of a few possible  answers:
>> 
>> 1) Only those triples defined by the mapping to be in the unnamed graph
>> 2) All of the triples defined by the mapping (including those produced in the unnamed graph as well as in named graphs)
>> 3) This is an implementation specific detail that R2RML does not address
>> 
> My position is that R2RML should not say anything about it. This will be decided by semantics of SPARQL (protocol and query language). In the scenario you mentioned, current SPARQL semantics says that the query will be executed against the default graph, which in a simple case consists of the set of triples in the unnamed graph.
>> A related issue is whether an implementation is allowed to add triples (either in the unnamed graph or a named graph). For example, can an implementation add triples that are metadata describing the relational database or the mapping itself. This would be roughly analogous to the "system" tables provided by relational databases.
>> 
> This is an interesting idea. I can see its use in specifying schema triples (such as, hierarchy and equivalence of properties and classes) that could be used by an implementation during SPARQL to SQL translation.

There was a slightly similar discussion in the RDFa Working Group. RDFa defines how certain attributes end up as triples in a graph, but an RDFa processor may also generate extra messages, eg, errors or warning. The decision in that group that a separate graph (we called it 'processor' graph) is supposed to receive those triplets, ie, they are separated from the triples coming from the RDFa content. The analogous situation here would be to say that there is an extra, and separate graph for the R2RML processor which is used for that purpose.

Ivan



>> -David
> Thanks,
> - Souri.
> 


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