Input To Today's DQL Discussion

From: Richard Fikes (
Date: 06/04/02

  • Next message: Peter F. Patel-Schneider: "Re: Input To Today's DQL Discussion"
    Attached below is an excerpt of a message I recently sent to Pat and
    Ian.  I don't expect you to have read it before the telecon, but I want
    you to have it so that I can refer to it in today's discussion of DQL.   
    I said earlier:
    > There are several issues being discussed together in the recent e-mails
    > and telecons regarding DQL that I think are separable.  One issue is the
    > definition of a minimal identifying description (MID) for a node in an
    > RDF graph.  A node's MID is independent of any particular use of the
    > MID.  In particular a node's MID is independent of its use in a query
    > answer.  So, we can decide on a definition for a MID independent of
    > DQL.
    We now actually have two definitions of a MID.  I think they are
    equivalent and that they formally define the graph that we have in
    mind.  So, even though Peter thinks they are silly, I think we now have
    MIDs if we want to use them.
    The more serious issue that Peter has raised, in my opinion, is
    independent of whether we use MIDs or not.  Here are three KBs taken
    from his examples that I think capture the essence of the problem:
    KB1   John rdf:type _:r .
          _:r daml:onProperty friend .
          _:r daml:minCardinality 1 .
    KB2   John friend _:f .
    KB3   John friend Bill .
    KB1 entails the sentence "(exists ?x (friend John ?x))", but does not
    explicitly contain that sentence.  KB2 explicitly contains that
    sentence.  KB3 explicitly names a friend of John.
    Consider asking the following query:
    > Query        John friend ?l .
    >      ?l distinguished
    We have been saying that query answers have to be entities that are
    explicitly referenced in the KB.  In KB1 above, there is no explicit
    reference to a friend of John's.  In KB2 and KB3, there is an explicit
    reference to a friend of John (i.e., _:f and Bill, respectively). 
    Therefore, according to the current DQL spec, the query would have no
    answers from KB1 and would have one answer from KB2 and one answer from
    Peter says
    > It seems exceedingly
    > strange that two equivalent KBs can give rise to different answers to a
    > query.  I view this a fatal problem that must be solved.

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