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Re: On the semantics of optional

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>
Message-ID: <20050920111706.GC4592@login.ecs.soton.ac.uk>

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 GMT

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