- From: Seaborne, Andy <andy.seaborne@hp.com>
- Date: Fri, 04 Aug 2006 14:23:14 +0100
- To: RDF Data Access Working Group <public-rdf-dawg@w3.org>
An acknowledgement has been sent. Some change will be needed - I haven't worked the example through on Fred's alternative proposal yet. Andy -------- Original Message -------- Subject: { } and OPTIONAL Resent-Date: Thu, 03 Aug 2006 17:38:20 +0000 Resent-From: public-rdf-dawg-comments@w3.org Date: Thu, 3 Aug 2006 13:34:15 -0400 (CLT) From: Jorge Pérez <jperez@ing.puc.cl> Reply-To: jperez@ing.puc.cl To: public-rdf-dawg-comments@w3.org <public-rdf-dawg-comments@w3.org> Hi! In rq24 in the definition of the Empty Group pattern the use of "requiring no substitution" is not clear (at least not for me). The specification of the example SELECT ?x WHERE { } is more clear but I think that it leads to an unexpected behavior. rq24 states that SELECT ?x WHERE { } "matches with one solution which has no substitutions for variables." Consider the data ex:a ex:p ex:b Does the definition mean that ?x -> ex:a is not a solution for the empty pattern { }? If this is the case then the pattern { } OPTIONAL { ?x ex:p ex:b } will not has as solution ?x -> ex:a because it is not a solution for the first pattern (applying the definition of OPTIONAL). This implies that for every pattern of the form { } OPTIONAL { P } no mather what P is, the solution is always the empty set. Is this the desired behavior? If it is not, I think that, to be consistent in the definitions, at least ?x -> ex:a must be consider a solution for { } with the dataset above and then (to be consisten again) ex:a must appear in the result of SELECT ?x WHERE { }. I don't know if I'm understanding well the definitions about { }, but I think that at least they deserve test cases with OPTIONALs and conjunctions of group graph patterns (for example a pattern like { { } { ?s ?p ?o } }). In the same context, what is the necessity of having an Empty Group Pattern? is there a Use Case or Requirement about that? - jorge
Received on Friday, 4 August 2006 13:23:24 UTC