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difference between XML and RDF Re: Classes in OWL

From: Charles McCathieNevile <charles@w3.org>
Date: Sun, 5 Sep 2004 01:22:34 -0400 (EDT)
To: Chaitanya Saragadam <go4chaitu_eng@yahoo.co.in>
Cc: www-rdf-interest@w3.org
Message-ID: <Pine.LNX.4.55.0409050054210.12347@homer.w3.org>

Very roughly, XML Schema and RDF let you define different things about your
elements.

In XML, you build a tree structure that you can put information into. The
relation between the elements is essentially just parent-child-sibling and to
understand what it means you can read the documentation included in the
Schema. (Being able to explicitly do this is why XML schema is better than
DTD for large schemas - you can reliably annotate a particular element in
Schema, whereas in DTD the best guess is usually to look for comments close
to the definition in the DTD, and hope you find something useful).

In RDF you can express datatypes pretty much like you do in XML and human
readable notes in a similar way to XML schemas.

You can also express relations (something is a sub-class or sub-property of
something else) between your elements and any other RDF terms anyone has
defined, and you can express domain and range constraints (sort of like
saying what the children of an element can be, although you're actually
describing what things a given relation or property applies to). You can do
this sometime after you started using the original schema, when you discover
a new source of information.

If you start by using RDF, then using OWL you can define more things about
the elements, to give you some more of the syntactic validation stuff you can
do with XML schema, while maintaining the possiblity to relate to other
vocabularies. Again, you can refine this as you go. OWL also has some useful
ways of describing versioning of your schema.

Note that if you use OWL DL or OWL Lite you can't use all of RDF Schema -
it's a trade-off between being able to guarantee that you can compute a
result for a query - what DL/Lite give - and having more expressive power to
deal with partial knowledge - what you get with OWL full, which is compatible
with all the other bits of RDF specifications.

There's a quick introduction to what RDF does in section 2 of the RDF Primer
- http://www.w3.org/TR/rdf-primer/#statements (and there are lots of others
around).

For more information on defining classes and subclasses, properties,
subproperties and the range and domain of properties I suggest looking at the
RDF Primer - http://www.w3.org/TR/rdf-primer/#rdfschema introduces them. That
section also gives some introduction to using datatypes - but these can be
more generally applied. Have a look at the relevant section of the RDF
Vocabulary Definition Language (http://www.w3.org/TR/rdf-schema - it used to
be called RDF schema) for more.

Using label and comment (and xml:lang to identify what language the labels
are actually in) is in my opinion very important. There's a little bit on
this in the Primer at http://www.w3.org/TR/rdf-primer/#otherschema and there
are pplenty of examples (see my post in your more recent thread -
http://lists.w3.org/Archives/Public/www-rdf-interest/2004Sep/0021.html - for
an example).

cheers

Chaals

On Tue, 31 Aug 2004, Chaitanya Saragadam wrote:

>What is the difference rdf and xml ,
>rdf schema versus xml schema.
>
> see,
>       suppose, there is an xml file
>  file1.xml:
>        <x1>
>          <x2>5345</x2>
>          <x3>
>             <x4>34534</x4>
>             <x5>"cieaflkd"</x5>
>          </x3>
>        </x1>
>  ok,
> now , the xml schema will define abt the elements and
>its datatypes or its so called sub-elements.
>
>But here Rdf/xml file has,
>         <rdf>
>            <person>
>               <name> Uri or value </name>
>                <age> uri or value </age>
>            </person>
>         </rdf>
> ok,
>     so where is the schema defined for the person
>here.And what are the datatypes of name and ages ,
>where shud i define them ,like the one in xml schema.
>Do i need to write one more rdf schema file defining
>them??
> Pls clarify the concept clearly.
>Thanx in advance,
>bbye.
>
>
>
>
>
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Charles McCathieNevile  http://www.w3.org/People/Charles  tel: +61 409 134 136
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Received on Sunday, 5 September 2004 05:22:34 GMT

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