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Abiguities in combination of group graph patterns

From: Jorge Pérez <jperez@utalca.cl>
Date: Wed, 19 Apr 2006 10:54:29 -0400
Message-ID: <1145458469.44464f25880a4@webmail.utalca.cl>
To: public-rdf-dawg-comments@w3.org

Hi! In the last draft, little attention is put on group graph patterns 
and "combinations" of group graph patterns. The draft says nothing about how 
the solution of

WHERE {
  { A . }
  { B . }
}

is. There is just an example in where A and B are triple patterns, in which 
case the above WHERE clause is the same as

WHERE {
  A .
  B .
}

but what if A or B are more complex graph patterns? It is not clear what the 
example is trying to say: 

1) does it say that {} (inside other {}) are useless and on can just omit 
them? and then {{ A. } { B. }} is alway equal to { A. B. }
2) Other possiblity of evaluation is to think that { A. } and { B. } are graph 
patterns in the same group graph pattern, aplying the group graph pattern 
formal definition, a solution to the pattern { { A. } { B. } } must 
*independently* match the patterns A and B.

what is the right? When looking in some implementations (SPARQLer and others) 
it seems that they are just omiting {}, i.e. using 1). following my example in 
http://lists.w3.org/Archives/Public/public-rdf-dawg-comments/2006Apr/0020.html
what would be the answer for

WHERE {
  { ?B :titlename ?N . }

  { ?B :year ?Y .
    OPTIONAL { ?B :authorname ?N } }
}

if one agree with 2) the solution must be the same as the solution for

WHERE {
  { ?B :year ?Y .
    OPTIONAL { ?B :authorname ?N } }

  { ?B :titlename ?N . }
}

but implementations gave me differents results...

Note that there may be more complex forms of combinations, so this issues 
deserves clarifications. If the editors agree with 1) then for simplicity the 
{} must be refused inside other {}, this is the most simple way to avoid the 
problem, for example

{ A. { B. { C. } { D. }} . OPTIONAL E. }} == { A. B. C. D. OPTIONAL E. }

If they agree with 2) (not the most simple but seems to be the more "natural") 
I think that some clarifications may be added to the draft.

- Jorge

-------------------------------------------------
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Received on Wednesday, 19 April 2006 14:58:49 GMT

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