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Re: What is an RDF Query?

From: David Allsopp <d.allsopp@signal.qinetiq.com>
Date: Wed, 12 Sep 2001 10:40:56 +0100
Message-ID: <3B9F2DA8.3000AAB7@signal.qinetiq.com>
CC: www-rdf-rules@w3.org


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*? 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.

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").

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...

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.

Regards,

David Allsopp

-- 
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Received on Wednesday, 12 September 2001 05:41:49 GMT

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