Re: DQL queries

From: Ian Horrocks (
Date: 07/23/02

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    I am sorry for the short notice, but I wont be able to make the call this
    evening. As you already saw from Frank, we are both attending ECAI in
    Lyon. I spent 5 hours today giving a talk, a tutorial and being
    interrogated about DAML+OIL/OWL, so while I would love to talk about DQL,
    I am afraid that have neither the energy nor the voice for it.
    I will answer by email the points Richard makes below, and then we can 
    hopefully discuss DLQ next week.
    Regards, Ian
    On Tue, 23 Jul 2002, Richard Fikes wrote:
    > > The current DQL spec says:
    > > 
    > > A DQL query contains a "query pattern" that is a collection of
    > > DAML+OIL sentences in which literals and/or resources have been
    > > replaced by variables.
    > > 
    > > As I mentioned in the telecon, I believe that it may be difficult to
    > > give sense/meaning to arbitrary sentences and replacements.
    > Given the ongoing concerns and discussions about the semantics of
    > DAML+OIL vis-a-vis its embedding in RDF, and given that the future is
    > OWL rather than DAML+OIL, I don't think this issue is worth much of our
    > energy.  Also, Pat's revision of the DQL spec attempts to finesse this
    > issue by allow servers to publish the class of query patterns that they
    > support.  If we agree on that revision, then I think the issue is mote.
    > I will simply state here that in our work with JTP, we have taken
    > seriously the notion that a DAML+OIL KB is a collection of RDF triples,
    > we use the axiomatic semantics for DAML+OIL, and we allow any element of
    > any triple in a query pattern to be a variable.  Given that query
    > answering is being defined as finding uri-refs or literals explicitly
    > mentioned as terms in the KB or defined as terms in DAML+OIL, the
    > semantics of such queries (at least based on the axiomatic semantics)
    > and of what constitutes an answer to such queries seems to be clearly
    > and precisely defined.
    > > Peter suggested range(P,v)
    > > and domain(P,v), but the semantics here are not quite so clear - are
    > > we asking about syntax, or do we mean something like "the biggest
    > > class C s.t. onProperty(P),toClass(NOT C)) is inconsistent? (I doubt
    > > that, in general, there exists a unique answer the latter kind of
    > > query; in fact such queries may very well be undecidable.)
    > I don't understand why the semantics are not clear.  The semantics of
    > "domain" and "range" are clear, and any class explicitly mentioned in
    > the KB that a reasoner can infer is a domain or range of P is an answer
    > to the query.  
    > > I meant to exclude query elements like onProperty(x,y), unionOf(x,y),
    > > which have doubtful/uninteresting semantics from a DAML+OIL point of
    > > view, and could be seen more as "syntax queries", i.e., queries
    > > regarding the syntactic content of the ontology.
    > The semantics of such triples is clear if one assumes that a restriction
    > is a class like any other class and that lists are objects in the domain
    > of discourse.  That's what we did in the axiomatic semantics and that's
    > what we do in JTP.  
    > > What I was trying to suggest was that some of the properties (and
    > > maybe some of the classes) in daml+oil.daml would not lead to such a
    > > clear semantic account of query element bindings. E.g., for
    > > unionOf(C,w), where C is some class, what would w bind to?
    > W would bind to any list explicitly mentioned in the KB.
    > > If queries
    > > are to have a well defined semantics, then we need to decide how to
    > > handle these cases.
    > That isn't a problem using the axiomatic semantics.
    > > There are some subtle problems here. E.g., if RDFS is used to generate
    > > a subProperty P of sameClassAs, how is P treated in DAML+OIL queries?
    > > According to the current (model theoretic) semantics, if I write
    > > P(C,D), then a DAML+OIL reasoner would NOT infer sameClassAs(C,D).
    > Well, it seems that sameClassAs(C,D) is the clear intention.  The
    > axiomatic semantics would sanction that inference.  I haven't been a
    > part of the debate of that issue, but at first blush, that sounds like
    > an inadequacy of the model theoretic semantics specification.
    > The bottom line here is this.  I don't expect you or the committee to
    > agree with the meaning attached to arbitrary triples in query patterns
    > with arbitrarily placed variables that is given to such queries by the
    > axiomatic semantics.  However, I hope that you agree such meaning and
    > such queries are coherent and perhaps even useful.  A major payoff to
    > allowing queries with arbitrary triples with arbitrarily placed
    > variables is that it greatly simplifies the specification of the query
    > language.  The relatively straightforward description of what a query is
    > and what an answer is that is in the current DQL specification is then
    > sufficient.
    > We can rejoin this discussion with respect to OWL when its specification
    > has been agreed on.
    > Richard
    Ian Horrocks, Department of Computer Science,
    University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
    Tel: +44 161 275 6133  Fax: +44 161 275 6211  Email:

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