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Re: str() function should also accept blank node argument

From: Andy Seaborne <andy.seaborne@epimorphics.com>
Date: Fri, 07 Oct 2011 09:49:08 +0100
Message-ID: <4E8EBD04.1040603@epimorphics.com>
To: public-rdf-dawg-comments@w3.org
David,

The following should carry out that task - delete everything and put one 
instance back.  Proceed by a query to check if it's needed.

----------------------
PREFIX : <http://example/>

DELETE { :x :p ?x .
          ?x :foo "a" ; :bar "b" . }
WHERE { :x :p ?x .
         ?x :foo "a" ;
            :bar "b" .
         FILTER( isBlank(?x) )
       }

INSERT DATA { :x :p [ :foo "a" ; :bar  "b" ] }
----------------------

Most of the results formats (XML, JSON, TSV) already have a per-result 
set bNode identifier but it is per-resultset and not the identifier used 
by the store to identify the bNode internally to the store (if there is 
a stable one at all).

Changing str() would only work if it were the store-wide identifier AND 
that identifier could be used in a later operation.  Some systems do 
provide this as a non-standard extension and have done since SPARQL 1.0.

The RDF-WG has published a revised RDF Abstract Syntax document which is 
relevant here as it describes bNode skolemization:

http://www.w3.org/TR/rdf11-concepts/#section-blank-nodes

	Andy

On 05/10/11 05:26, David Booth wrote:
> http://www.w3.org/TR/sparql11-query/#func-str
> The str() function is currently defined to accept only a literal or an
> IRI  -- not a bnode.  Is there a compelling reason why not?  I think
> this is a problem because AFAICT it is currently impossible to impose an
> ordering on bnodes, since the "<" operator also does not compare IRIs or
> bnodes.  I think the str() function should also accept a bnode argument
> and return a lexical form of that bnode that is unique to the result
> set.  This would allow one to write a query that distinguishes the
> "first" bnode from others (where the choice of first is arbitrary, but
> holds within the query), thus allowing that query to (for example)
> delete all "non-first" bnode objects.
>
> For example, consider the following RDF:
>
>    :x :p [ :foo "a" ; :bar "b" ] .
>
> When it is read in repeatedly to a graph, each time it is read in an
> additional bnode will be created, thus leading to multiple (redundant)
> statements in the graph, each having a *different* bnode:
>
>    :x :p _:b1 .      _:b1 :foo "a" ; :bar "b" .
>    :x :p _:b2 .      _:b2 :foo "a" ; :bar "b" .
>    :x :p _:b3 .      _:b3 :foo "a" ; :bar "b" .
>      . . .
>
> I.e., the graph becomes increasingly non-lean.  It would be nice to be
> able to write a simple DELETE query to get rid of these extra bnode
> objects, but without the ability to impose an arbitrary ordering on them
> in the query, I do not see an easy way to distinguish one bnode object
> from all the others and delete only "all the others".  In the case where
> there is a known, fixed number of predicates in the bnode object then I
> could use a nested SELECT DISTINCT, but I want to be able to write the
> query more generally, where the set of predicates is not fixed (though
> the depth of the bnode object would be fixed, to avoid getting into the
> general problem of making graphs lean).
>
> There are probably other use cases that would also benefit from the
> ability to impose an ordering on bnodes -- just as it is useful to be
> able to impose an ordering on other entities -- but this is the case
> that happened to motivate me to send in this suggestion.
>
> I assume that this suggestion is too late for consideration in SPARQL
> 1.1, but if it could be added to a wish list for future consideration I
> would appreciate it.
>
> Thanks!
>
Received on Friday, 7 October 2011 08:49:38 GMT

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