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Re: what RDF is not (was ...)

From: Graham Klyne <GK@ninebynine.org>
Date: Thu, 03 Jan 2002 12:53:35 +0000
Message-Id: <>
To: Sandro Hawke <sandro@w3.org>
Cc: www-rdf-interest@w3.org

I think there's a lot of justification for this viewpoint in John Sowa's 
book Knowledge Representation.  Throughout the book, he goes to some 
lengths to show the expressive equivalence of various notations, including 
FOL, CGs, Petri nets, and more.  In particular, the tables of a relational 
database are shown to be equivalent to the existential-conjunctive (EC) 
subset of first order logic (i.e. what can be said with EXISTS, AND and 
n-ary predicate expressions).  RDF is similar, except that predicates are 
limited to binary form.

(Nit.  I think the comparison between RDF and SQL slightly misses the 
point:  SQL is a query language.  I'd say the appropriate comparison is 
between RDF and relational tables.)

And I agree with the (implied) conclusion, that RDF is a useful building 
block, sufficient for some (limited) purposes, but certainly not for all.

As for the difference between RDF and XML, I'd say the crucial one is that, 
lacking semantics, XML has no concept of entailment.  (PFPS' recent work 
may change this.)


At 12:18 PM 1/2/02 -0500, Sandro Hawke wrote:

>Here's my little rant on what RDF is.   Not directed at anyone in
>RDF is a language for transmitting pieces of collaborative databases.
>It started as a way to categorize web pages, but since the subject
>matter of the web is arbitrary, RDF ended up as a way to express
>arbitrary information, just like one might store in a relational DBMS.
>The pieces of RDF are peices of a web-wide database of information,
>not just about web pages but about anything.
>While SQL is a database manipulation and query language, RDF is just a
>data format, equivalent to the tables that result from a SQL query or
>to an on-disk database file format.  (RDF still needs a SQL-equivalent
>language.)  RDF's database model is different from SQL's in being
>"webized" to support distributed collaboration: tables/columns and
>datatypes are named in a global namespace (URIs) so they can be
>automatically linked.
>There is a temptation to think a mass of RDF fragments can store all
>of human knowledge.  The truth is that RDF is only marginally better
>than a typical SQL database for storing "knowledge".  It works well
>for a catalog of the CDs you own, or the products you sell, or the
>configurations of software installed on your computers, but the only
>thing it does for "knowledge representation" and "machine reasoning"
>is provide a standard underlying format.
>(If RDF sounds a lot like XML, well, it is.  The difference is that an
>XML database fragment is less self-describing than an RDF one.
>Whether this difference is critical is a subject of debate.  Whether
>either of them is better than a comma-separated-values file is also
>subject to debate.  The basic question is whether self-description is
>So how do you encode some knowledge like "All men are mortal" or "Only
>3 Sale-Items Per Customer" in RDF?  The same way you do in SQL: you
>don't.  You need another mechanism - some logic somewhere else in the
>system.  It may, however, be a standard logic, driven by information
>also in the database.  That is, the database can hold software written
>in some programming or constraint language (Perl, Python, i386 machine
>code, first-order predicate logic, DAML, RDFS, etc), and there can be
>conventions about how apply that knowledge to other knowledge in the
>database (eg for database validation or inference).
>Putting other-language elements into a database like this is a common
>design style for complex database applications.  Additionally,
>database systems which do validation or inference (as many of them now
>do) often make available data views of the logic-language expressions.
>It's a fairly obvious technique.  RDF may cloud the issue by
>encouraging a different encoding style, where you encode each
>logic-language token (instead of each whole ASCII expression) in
>separate RDF objects.  This somehow makes the language look more like
>it's "in" RDF or extending RDF; the truth is, for RDF, it's only data
>in the database.
>      -- sandro                 http://www.w3.org/People/Sandro/

Graham Klyne
Received on Thursday, 3 January 2002 08:18:38 UTC

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