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RE: ISSUE-3: REPORTED: Lack of anonymous individuals

From: Boris Motik <boris.motik@comlab.ox.ac.uk>
Date: Fri, 9 Nov 2007 10:50:14 -0000
To: "'Giorgos Stoilos'" <gstoil@image.ece.ntua.gr>
Cc: <public-owl-wg@w3.org>
Message-ID: <001101c822be$4f41c4a0$2711a8c0@wolf>

Hello,

You can use bnodes in RDF data arbitrarily. Take a look at

http://www.w3.org/TR/rdf-primer/#structuredproperties

There, you'll see triples containing identifies form _:xxx; all of these are bnodes. Each such identifier is taken to represent one
existentially quantified variables.



The bnodes in RDF are the same as labelled nulls in databases. Database people have studied in depth what kind of semantics is
appropriate for null values. A really good paper on this topic is the following:

Tomasz Imielinski, Witold Lipski Jr.: Incomplete Information in Relational Databases. J. ACM 31(4): 761-791 (1984)

The practical consequences, however, are rather severe: answering queries with labelled nulls is NP-complete. This is one of the
main reasons why practical database systems don't implement labelled nulls. (Another reason is that your answers are not so much
better even if you use labelled nulls.)

Boris 


> -----Original Message-----
> From: Giorgos Stoilos [mailto:gstoil@image.ece.ntua.gr]
> Sent: 09 November 2007 10:18
> To: 'Boris Motik'
> Cc: public-owl-wg@w3.org
> Subject: RE: ISSUE-3: REPORTED: Lack of anonymous individuals
> 
> 
> 
> > -----Original Message-----
> > From: Boris Motik [mailto:boris.motik@comlab.ox.ac.uk]
> > Sent: Thursday, November 08, 2007 2:14 PM
> > To: gstoil@image.ece.ntua.gr; public-owl-wg@w3.org; 'Carsten Lutz'
> > Subject: RE: ISSUE-3: REPORTED: Lack of anonymous individuals
> >
> > Hello,
> >
> > Here is an explanation how anonymous individuals in ABoxes correspond to
> > conjunctive queries. I will use a "pidgin" LaTeX
> > first-order logic notation for this. I'll use _:x for anonymous
> > individuals, and I'll use != for inequality (DifferentFrom)
> > assertions and & for conjunction.
> >
> >
> > Imagine you have an ABox A containing the following assertions:
> >
> > (1)  hor(_:1,_:2)
> > (2)  ver(_:2,_:3)
> > (3)  ver(_:1,_:4)
> > (4)  hor(_:4,_:5)
> > (5)  _:3 != _:5
> >
> > Under the traditional semantics, anonymous individuals are actually
> > existentially quantified variables. Hence, the ABox A is
> > actually equivalent to the following first-order formula \varphi:
> 
> Hello,
> 
> But is it possible to have the above statements in OWL 1.1? Since _:3 and
> _:5 are anonymous how can you refer to them in this difference assertion
> (!=)?
> 
> Greetings,
> -gstoil
Received on Friday, 9 November 2007 10:51:07 GMT

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