Re: “Default” vs “unnamed” graph

Hi Eric,

Maybe I'm misinterpreting your 'desiderata chain' below, but I don't think
I'd agree that "SPARQL doesn't say whether the first FROM supplements or
replaces the default graph.".  The
spec<http://www.w3.org/TR/sparql11-query/#rdfDataset>clearly states "A
SPARQL query may specify the dataset to be used for matching by using the
FROM clause and the FROM NAMED clause to describe the RDF dataset. If a
query provides such a dataset description, then it is *used in place of* any
dataset that the query service would use if no dataset description is
provided in a query. "

My apologies if I'm reading something out of context.

Bob Scanlon
Revelytix


On Tue, Mar 15, 2011 at 4:49 PM, Eric Prud'hommeaux <eric@w3.org> wrote:

> We talked for a while and settled on "default". Desiderata chain:
>
>  SPARQL doesn't say whether the first FROM supplements or replaces
>  the default graph. Any protocol which addresses this is independent
>  of (has broader applicability than) RDB2RDF. Any such protocol will
>  be phrased in terms of the "default graph" and not in terms of the
>  "unnamed graph". If R2RML emits an "unnamed graph", it won't have a
>  defined semantics in SPARQL (know one will know how to query the
>  "unnamed graph"). If we use "unnamed graph", we'll have to provide
>  the linkage to "default graph". That recipe, plus the identifier
>  "unnamed graph" won't give us any more versatility than we already
>  have.
>
> Apparently I was mistaken about the choice of replacing or
> supplementing, which means that I know of several non-conferment
> implementations. I don't think that this changes the resolution,
> do you Seema or Souri?
>
>
> * Richard Cyganiak <richard@cyganiak.de> [2011-03-15 21:18+0000]
> > I'd like to re-iterate my position from this call that we should define
> the output of an R2RML mapping as an RDF Dataset in the SPARQL sense, as it
> already says in the introduction, and consistently use the SPARQL's
> terminology.
> >
> > This would imply using the terms “named graph” and “default graph”. The
> term “unnamed graph” would be removed from the spec.
> >
> > The objection raised in the call was that the default graph used in a
> SPARQL query can actually be constructed on the fly, on a query-by-query
> basis, by using the FROM keyword or SPARQL protocol parameters.
> >
> > This is a valid observation. But I argue that this doesn't conflict at
> all with the use of the RDF Dataset concept and the term “default graph”.
> >
> > To quote from the SPARQL spec [1]:
> >
> > > A SPARQL query may specify the dataset to be used for matching by using
> the FROM clause and the FROM NAMED clause to describe the RDF dataset. If a
> query provides such a dataset description, then it is used in place of any
> dataset that the query service would use if no dataset description is
> provided in a query.
> >
> > This makes clear that if FROM/FROM NAMED are used, then one queries a
> *different* dataset from the one that the query service offers *by default*
> if FROM/FROM NAMED were not used.
> >
> > I'm proposing that we think of the R2RML-generated dataset as the dataset
> which a query service would use by default in absence of a specific dataset
> description. This doesn't preclude the possibility of overriding the default
> graph or any other graph with FROM/FROM NAMED and the SPARQL protocol.
> >
> > This would be a simple change in terms of spec text (s/unnamed
> graph/default graph/ and check the early sections for anyplace that should
> say “RDF dataset” instead of “RDF graph”). So I propose that we do this
> before the WD release.
> >
> > If there are no objections (on- or off-list), I'll go ahead and do this.
> >
> > Best,
> > Richard
> >
> > [1] http://www.w3.org/TR/rdf-sparql-query/#unnamedGraph
>
> --
> -ericP
>
>