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Re: graph keyword and exists and scope

From: Andy Seaborne <andy@apache.org>
Date: Tue, 21 Jun 2016 14:51:18 +0100
To: public-sparql-dev@w3.org
Message-ID: <57694656.1060706@apache.org>
On 17/06/16 20:36, james anderson wrote:
>
>> On 2016-06-17, at 21:10, Jeremy J Carroll <jjc@syapse.com
>> <mailto:jjc@syapse.com>> wrote:
>>
>> Given the interesting conversation about EXISTS, I have a slightly
>> related question
>>
>>
>>  select *
>> { graph <eg:graph> {
>>       ?s <eg:b> ?o
>>       FILTER EXISTS {
>>           ?s <eg:c> ?o
>>      }
>>  }
>> }
>>
>>
>> is the <eg:c> matched against the default graph or <eg:graph>?
>
> given the definitions in the recommendation, {?s <eg:c> ?o } should
> apply to the given graph.
>
> while the definition does not take the trouble to describe the meaning
> specific to forms, i have understood only
> - {graph <x>  {graph <y> … } } to limit the scope
> - {graph ?x … } to have scope limited by projection which did not include ?x
> - {graph ?? { service … } } to limit scope
>
> where, for the service case, i am ambivalent.
>
>>
>> I sometimes get results for this sort of query that surprise me, and
>> so I tend to explicitly repeat the graph scope.


In the definition:
https://www.w3.org/TR/sparql11-query/#defn_evalGraph

   if IRI is a graph name in D
   eval(D(G), Graph(IRI,P)) = eval(D(D[IRI]), P)

   if IRI is not a graph name in D
   eval(D(G), Graph(IRI,P)) = the empty multiset

   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 is setting the active graph from "GRAPH <eg:graph>" or "GRAPH ?g" 
to the {contained} pattern all the way down.  No, you can't get back to 
the default graph - the both WGs were aware of this as I recall.

For SERVICE, all that matters is the endpoint - the active graph is not 
passed across.

	Andy

>
> best regards, from berlin,
> ---
> james anderson | james@dydra.com <mailto:james@dydra.com> | http://dydra.com
>
>
>
>
>
Received on Tuesday, 21 June 2016 13:52:00 UTC

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