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Re: Querying Named Graph using SPARQL

From: Andy Seaborne <andy@apache.org>
Date: Wed, 14 Aug 2013 11:27:04 +0100
Message-ID: <520B5B78.7010206@apache.org>
To: public-sparql-dev@w3.org
The evaluation of GRAPH is defined by

http://www.w3.org/TR/sparql11-query/#defn_evalGraph

eval(D(G), Graph(var,P)) =
      Let R be the empty multiset
      foreach IRI i in D
         R := Union(R, Join( eval(D(D[i]), P) , Ω(?var->i) )
      the result is R

so it's a loop over

Join(
    eval on graph_i of pattern {}
    Ω(?var->i)
)

eval on graph_i of pattern {} is a binding od one row, no variables; the 
join identity.  Joined to a binding of ?var/i and you get ?var/i.

Union it all and it's the set of graph IRIs (no duplicates because the 
join identity does not introduce any).

	Andy

On 13/08/13 20:10, Axel Polleres wrote:
>> [[
>> The group pattern:
>>   { }
>> matches any graph (including the empty graph) with one solution that does not bind any variables.
>> ]]
>
> This only means that upon
>
>   SELECT ?G WHERE { GRAPH ?G {} }
>
> also empty named graphs should be returned, which would not be the case for
>
>   SELECT ?G WHERE { GRAPH ?G { ?S ?P ?O } }
>
> Obviously, this makes a difference for all graph stores that support empty named graphs.
> So, to my understanding at least, this is not a bug in the spec.
>
> HTH,
> Axel
>
> On 13 Aug 2013, at 20:21, David Booth <david@dbooth.org> wrote:
>
>> Hi Axel,
>>
>> That doesn't work in Sesame 2.7.1 at least, apparently because ?G is not bound, even though there is one solution.  The SPARQL 1.1 spec says:
>> http://www.w3.org/TR/sparql11-query/#emptyGroupPattern
>> [[
>> The group pattern:
>>   { }
>> matches any graph (including the empty graph) with one solution that does not bind any variables.
>> ]]
>>
>> Is this a bug in the spec?
>>
>> David
>>
>> On 08/13/2013 11:48 AM, Axel Polleres wrote:
>>> Hi Quentin,
>>>
>>> how about just
>>>
>>>   SELECT ?G WHERE { GRAPH ?G {} }
>>>
>>> (no need to dump all triples, if the only concern is which ?G exist)
>>>
>>> BTW, public-sparql-dev@w3.org may be the list you wanted to use.
>>>
>>> best,
>>> Axel
>>>
>
>
Received on Wednesday, 14 August 2013 10:27:32 UTC

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