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Re: What is an RDF Query?

From: Eric Prud'hommeaux <eric@w3.org>
Date: Thu, 13 Sep 2001 16:27:53 -0400
To: David Allsopp <d.allsopp@signal.QinetiQ.com>
Cc: www-rdf-rules@w3.org
Message-ID: <20010913162753.D27938@w3.org>
On Wed, Sep 12, 2001 at 10:40:56AM +0100, David Allsopp wrote:
> 
> 
> pat hayes wrote:
> 
> > OK, I took a look at http://lists.w3.org/Archives/Public/www-rdf-rules/#terms
> > and found this:
> > ----
> > rule
> >         Expression of query and assertion that causes the automatic
> > insertion of asserted statements wherever the query is matched.
> > -----
> > which may help to explain the (my) confusion. This notion of 'rule'
> > doesn't make sense to me. Why would matching a *query* produce the
> > insertion of *statements*? 

Be they in the original dataset or in temporary database from which
the query response is culled, the consequence of a rule must be noted
in order to be useful. I called that operation "inserting", perhaps
wrongly (or incoherently). By the terms I used,
  if P then Q
can be described as a rule made of a query P and an assertion Q.

>                              If we were to use a rule (in the usual
> > sense, now, not Eric's) in forward inference, then it would match an
> > assertion and produce a new assertion (as in inferring Q from P and
> > (if P then Q) ) , and if we are using it in backwards mode then it
> > would match a query and produce a new query (as in  going from ?Q and
> > (if P then Q) to ?P). These are really the 'same' inference,
> > logically speaking - modus ponens - but done in different
> > 'directions'. But I can't think of any way to take a rule, match it
> > to a query, and get an assertion.

In my terms, that would be
  (query1 + assertion1) + query2 => assertion2.
  |------ rule1 ------|  |------ rule2 -------|

I agree, I don't see this as useful except in doing rules about rules
which would be used proof checking or other mechanisms to control the
behaviour of an inferencing engine. Even there, I don't believe that
rule1 is treated like a rule, ie. evaluated, so much as considered as
a set of statements in matching query2.

> No, nobody's saying one should match a _rule_ against a _query_, I
> think.
> 
> As Pat says, the antecedent of a rule matches assertions. I think some
> people regard the antecedent as a query over the fact base ("Is this
> assertion in the fact base? If so, assert the consequent").

I meant the query and assertion to be tied together. I state a rule in
terms of a query and set of assertions.

Looking for queries to match rules, as alluded to above, seems like a
way one would optimize a backward chaining query to limit the working
set:
  dataset has a statements and two rules:
    P
    if P then Q
    if Q then R
  but has not done forward chaining. The query
    R?
  causes the inferencer to look for R in the dataset. The only
  candidate is the rule "if Q then R" so it looks for Q in the
  dataset etc.
I would say this is a perfectly reasonable strategy, but I was only
trying to characterize those rules "if P then Q" and "if Q then R"
in terms that would elucidate the parallels between "if P", "if Q"
and "R?".

> In implementation terms, the antecedent may literally involve a query,
> because to match it against an assertion one has to extract assertions
> from a fact base (i.e. query a database looking for matching
> assertions). One might then speak of "matching the query" i.e.
> succeeding in matching the rule's antecedent with an assertion.  I think
> this is common (though clearly confusing) usage...

I think the reason it is/could be useful is that the query part of
rules can be expressed in the same language as the query part of
queries (which would be the greater part).

> Perhaps the confusion lies in "query" as an expression (where some
> syntax, such as a question mark, indicates a question), and "query" as a
> implicit process, where assertions are compared.

Is there a difference beyond the expectation of what to do with the
results?

-- 
-eric

(eric@w3.org)
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Received on Thursday, 13 September 2001 16:27:54 GMT

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