From: Enrico Franconi <franconi@inf.unibz.it>

Date: Tue, 20 Sep 2005 00:55:12 +0200

Message-Id: <2f5144759ca71dc50ce633a6a9331635@inf.unibz.it>

To: RDF Data Access Working Group <public-rdf-dawg@w3.org>

Date: Tue, 20 Sep 2005 00:55:12 +0200

Message-Id: <2f5144759ca71dc50ce633a6a9331635@inf.unibz.it>

To: RDF Data Access Working Group <public-rdf-dawg@w3.org>

ON THE SEMANTICS OF OPTIONAL. The semantics of OPTIONAL as specified in the document, if we assume a relational-like algebra, should be more precisely defined as follows: QA OPTIONAL QB is defined as: (QA . QB) UNION (QA \setminus \project_{V-QA}(QA . QB)) where \project_{V-QA}(Q) is the projection of the solutions of Q over the variables of QA. We can show that it is possible to give a standard relational algebra account of the whole SPARQL (excluding, of course, the GRAPH parts; and leaving the minimisation routine and the CONSTRUCT outside the algebra.). OUR CONCLUSIONS We are not proposing a different semantics for OPTIONAL, but just a better version of it as an algebraic operator. The "parallel" optional construct needs still to be formalised.Received on Monday, 19 September 2005 22:58:24 UTC

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