W3C home > Mailing lists > Public > public-rdf-dawg@w3.org > July to September 2004

Re: More on additional semantic information

From: Jim Hendler <hendler@cs.umd.edu>
Date: Tue, 13 Jul 2004 22:56:07 -0400
Message-Id: <p0611041bbd1a4f381d91@[]>
To: Enrico Franconi <franconi@inf.unibz.it>, public-rdf-dawg@w3.org

  I'm not really sure how to reply except to say I simply don't agree 
- the problem is the points you make are true against some sort of 
"general" ontological framework, but not against OWL where there is a 
normative document, approved by the W3C as a web recommendation, that 
outlines the specific semantics in question.  I'm also very confused 
why you think that RDFS is somehow cleaner than OWL Full.  You are 
right that within the DL community there could be some issues, but 
the exact set of what is and isn't entailed in OWL is not up for 
argument (in OWL DL) and in OWL Full, I don't see where it is 
different than RDF-S.
  However, please be clear what the "additional semantic information" 
objective is about (as best I can tell) - it doesn't say create an 
OWL query language, it says where there is an RDFS or OWL ontology 
represent in the RDF graph that is being queried, there should be an 
ability to expose some or all of those semantics -- a simple example 
-- in a typical database if I query for
  ?x a ex:animal
I will only see those things that are exactly known to be animals -- 
but if Bob is a bear (and bear is an animal), I wouldn't be 
legitimized to answer "Bob" -- however, if I were querying an RDFS or 
OWL backend with some inferencing, I would be very surprised if it 
didn't include Bob.  I'd at least want a way to make it clear in the 
protocol, if not in the query proper, as to whether I would want to 
include inferred relations of some type.
  Look, a lot of us spent several years designing OWL to sit correctly 
on RDF, with the goal being, at least for some of us, that the 
ontological properties created become first class citizens of the Web 
-- for the DAWG to ignore this seems crazy to me.
  So, while I would actually like "additional Semantic Information" to 
be a requirement (not an objective), I can live with it as an 
objective.  However, for it to be completely ignored would seem to me 
we are not doing our job of supporting the entire RDF using 
population, which includes us RDFS and OWL users.
  -Jim H

At 23:53 +0200 7/12/04, Enrico Franconi wrote:
>Hi everybody.
>I'd like to make my first contribution in the discussion about the 
>handling of additional semantic information to a RDF-based query 
>language. I don't wont to enter into the debate of the wording of it 
>:-) but I want to give a technical contribution, which may help (or 
>may not).
>First of all, I'd like to point you to my recent paper [1], where we 
>try to give a formal unified description of the different notions of 
>rules and queries in the ontological world. I came late into the 
>discussion of this group since I wanted first to conclude this 
>unifying survey of the literature. What is relevant here is the 
>important border (Theorem 3) that divides the querying of a pure 
>database (say, a RDF graph or a completed RDFS graph) from the 
>querying of a database given a background ontology. We have shown 
>that the divide is quite relevant in several aspects:
>1) In the case of simple query languages (like conjunctive queries), 
>the absence of a background ontology makes all the different 
>semantics to queries given in the literature equivalent. This is to 
>be contrasted to the case of expressive query languages, and/or of a 
>background ontology which requires to choose an appropriate 
>semantics to the query. There are at least three different semantics 
>to a query which give rise to different answers (axiom-based, 
>logic-programming based, autoepistemic based).
>2) In the case of simple query languages (like conjunctive queries), 
>the absence of a background ontology makes query answering 
>computable as model checking: i.e., it is possible to check the 
>validity of the query over a *unique* form of completion of the 
>database with some sort of "matching" procedure; this is not true in 
>the presence of a background ontology (see example below).
>The points above are rather technical, but the conclusion to me is 
>that we should cross that border with extreme care. Basically, if we 
>restrict ourself to a simple query language like the one of 
>conjunctive queries, allowing for a non-trivial background ontology 
>(such as, e.g., an ontology written in OWL-lite) destroys the 
>"minimal model property" of the system. This means that a query 
>answering procedure considering only such a minimal model (this 
>would be for example the closure of a RDFS graph) can not work 
>anymore, and reasoning procedures should involve in a clever way all 
>the models (exponentially or possibly infinitely many) of the 
>system: that is deduction wrt model checking. Of course this is not 
>impossible, but of course the technologies involved will be 
>completely different and they can not be based on some sort of 
>"matching", like it happens now for the RDF(S) query answering 
>The way I understand it, the reason why all the work that has been 
>done in querying RDF(S) databases could be based on some notion of 
>matching, is that it is possible to compute the *unique* completion 
>(i.e., the deductive closure) over which then the validity of 
>queries is checked. This is not true anymore even for slightly more 
>complex frameworks.
>Let me make an example (forgive me the sloppyness in the notation, I 
>am not yet used to RDF stuff, I will certainly learn it discussion 
>after discussion in this mailing list). This example is a very old 
>example (at least 13 year sold) in the DL community; it is called 
>"the little house" example.
>the class WORKER is declared equivalent to the union of the classes 
>Paul rdf:type WORKER
>Andrea rdf:type WORKER
>Simon rdf:type EMPLOYEE
>Caroline rdf:type MANAGER
>Paul ns:has-friend Andrea
>Paul ns:has-friend Simon
>Simon ns:has-friend Andrea
>Andrea ns:has-friend Caroline
>The query:
>Tell me the workers having a friend which is an employee, which in 
>turn should have a friend which is a manager.
>[I don't know the syntax you prefer; in prolog-like notation this would be:
>  q(X) :- worker(X), has-friend(X,Y), employee(Y), has-friend(Y,Z), 
>The answer is the set {Paul}.
>To understand why Paul satisfies the query, you can first have a 
>visual representation of the database (the "little house") - all the 
>edges are of type "has-friend":
>          Paul:WORKER
>          /          \
>         /             \
>        V               V
>Andrea:WORKER <- - Simon:EMPLOYEE
>        |
>        |
>        V
>If you look for a path WORKER-EMPLOYEE-MANAGER, you don't find one. 
>However, you can say that in any possible world Andrea is either a 
>manager, or an employee, or both (since he/she is a worker, given 
>the ontology). Therefore, in each possible world you can find that 
>the node with Paul can be the root of a path 
>WORKER-EMPLOYEE-MANAGER. So, Paul is in the answer set of the query.
>The main thing that this simple example shows is that it is 
>impossible to find a unique completion (a unique deductive closure) 
>over which the query can be evaluated. There are *three* 
>incompatible completions, i.e., none of them is minimal (i.e., none 
>of them is included in all the others).
>A real reasoning by cases mechanism should be employed to answer this query.
>I don't know whether this helps the discussion.
>Enrico Franconi                  - franconi@inf.unibz.it
>Free University of Bozen-Bolzano - http://www.inf.unibz.it/~franconi/
>Faculty of Computer Science      - Phone: (+39) 0471-016-120
>I-39100 Bozen-Bolzano BZ, Italy  - Fax:   (+39) 0471-016-129
>[1] Enrico Franconi and Sergio Tessaris. Rules and Queries with 
>Ontologies: a Unified Logical Framework. Workshop on Principles and 
>Practice of Semantic Web Reasoning (PPSWR-2004), St. Malo, France, 
>2004; supported by the REWERSE and the CoLogNet European Networks of 

Professor James Hendler			  http://www.cs.umd.edu/users/hendler
Director, Semantic Web and Agent Technologies	  301-405-2696
Maryland Information and Network Dynamics Lab.	  301-405-6707 (Fax)
Univ of Maryland, College Park, MD 20742	  240-277-3388 (Cell)
Received on Tuesday, 13 July 2004 22:56:50 UTC

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