- From: Steve Harris <S.W.Harris@ecs.soton.ac.uk>
- Date: Tue, 20 Sep 2005 12:17:06 +0100
- To: RDF Data Access Working Group <public-rdf-dawg@w3.org>
On Tue, Sep 20, 2005 at 12:55:12 +0200, Enrico Franconi wrote: > > 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)) Is . in this expression shorthand for the join operator? If so, why not QA ]X| QB (where ]X| is the left outer join) I believe this has the same semantics, as OPTIONAL is not expicitly parameterised. This matches my SQL mapped implementation, as described here: http://eprints.ecs.soton.ac.uk/11126/01/harris-ssws05.pdf (specifically page 9) - Steve
Received on Tuesday, 20 September 2005 11:17:38 UTC