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One more question on UNSAID/MINUS

From: Axel Polleres <axel.polleres@urjc.es>
Date: Mon, 11 Dec 2006 09:09:39 +0100
Message-ID: <457D1243.5080401@urjc.es>
To: "Eric Prud'hommeaux" <eric@w3.org>
CC: axel@polleres.net, public-rdf-dawg-comments@w3.org, public-rdf-dawg@w3.org

Eric Prud'hommeaux wrote:
>>* I'd like to suggest to the working group some straightforward 
>>extensions of SPARQL such as adding the set difference operator MINUS, 
>>and allowing nesting of ASK queries in FILTER expressions which come 
>>basically for free in the approach.
> 
> 
> This appears to be an implementation of UNSAID, which the WG has
> decided to phrase as (for example):
>   OPTIONAL { ?x dc:created ?created } .
>   FILTER ( !bound(?created)) }
>   http://www.w3.org/2001/sw/DataAccess/tests/#dawg-unsaid-002
> 
> Please examine the "unsaid" issue description
>   http://www.w3.org/2001/sw/DataAccess/issues#unsaid
> and advise us if you have novel information, or if you are satisfied
> that the WG has discussed these points. You may close this thread by
> responding to this messasge, prefixing the subject with [CLOSED].

Allow me a final question befroe doing so:

As you write it, UNSAID (or MINUS) is already there, "encodable" by 
OPTIONAL and !bound. However, as Jorge's relational algebra shows UNSAID 
should rather be a part of OPTIONAL, i.e. set difference is more down to 
rel. algebra than OPTIONAL, which is then constructible as

(A OPTIONAL B)   =  (A AND B) UNION (A MINUS B).

It just seems more natural for me to express set difference directly.

best regards,
Axel



-- 
Dr. Axel Polleres
email: axel@polleres.net  url: http://www.polleres.net/
Received on Monday, 11 December 2006 08:18:59 GMT

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