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Re: Proposed change to the OWL-2 Direct Semantics entailment regime

From: Bijan Parsia <bparsia@cs.man.ac.uk>
Date: Mon, 20 Dec 2010 17:30:36 +0000
Cc: SPARQL Working Group <public-rdf-dawg@w3.org>, Maurizio Lenzerini <lenzerini@dis.uniroma1.it>
Message-Id: <0C5629BC-0083-4332-B00B-E0BD85DC5A8F@cs.man.ac.uk>
To: Enrico Franconi <franconi@inf.unibz.it>
Hi Maurizio,

Thanks for the message!

On 20 Dec 2010, at 09:48, Enrico Franconi wrote:

> I forward this message I received from Maurizio Lenzerini.
> Begin forwarded message:
>> In all the applications mentioned above, there is a strong need of answering queries with non-distinguished variables. Just to name one interesting scenario where missing non-distinguished variables would be a real problem, consider checking quality/completeness of data.
>> The query:
>> { x,z | R1(x,y), R2(y,z) }
>> tells me which x and z are connected through y, without necessarily knowing who is the y. On the other hand, the query
>> { x,y,z | R1(x,y), R2(y,z) }
>> tells me for which x,z I KNOW the y.

As an example, this isn't really very informative. It's a fake toy example which merely illustrates the difference between the two. We're all well aware of the semantic differences --- we're trying to find real cases we can study. 

And I seem to be missing something: Shouldn't a *data* completeness scenario be covered by the second query (perhaps with projection)? That is, for the data to be *complete*, don't I need it to be the case that x and z are connected via a bindable name? If the connection is merely existential then the data is incomplete, right? Or am I missing something?

If I just don't want to see the y, then leaving y ranging over the active domain is fine: Projection drops that column.

Thanks for your help!

Received on Monday, 20 December 2010 17:31:11 UTC

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