Chapter3 - Chapter 3 Describing Web Resources in RDF...

Chapter 3 A Semantic Web Primer
1
Chapter 3
Describing Web Resources in RDF
Grigoris Antoniou
Frank van Harmelen

Chapter 3 A Semantic Web Primer
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Lecture Outline
1.
Basic Ideas of RDF
2.
XML-based Syntax of RDF
3.
Basic Concepts of RDF Schema
4.
Τhe Language of RDF Schema
5.
The Namespaces of RDF and RDF Schema
6.
Axiomatic Semantics for RDF and RDFS
7.
Direct Semantics based on Inference Rules
8.
Querying of RDF/RDFS Documents using SPARQL

Chapter 3 A Semantic Web Primer
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Drawbacks of XML

XML is a universal metalanguage for defining
markup

It provides a uniform framework for interchange of
data and metadata between applications

However, XML does not provide any means of
talking about the semantics (meaning) of data

E.g., there is no intended meaning associated with
the nesting of tags
–
It is up to each application to interpret the nesting.

Chapter 3 A Semantic Web Primer
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Nesting of Tags in XML
David Billington is a lecturer of Discrete Maths
<course name="Discrete Maths">
<lecturer>David Billington</lecturer>
</course>
<lecturer name="David Billington">
<teaches>Discrete Maths</teaches>
</lecturer>
Opposite nesting, same information!

Chapter 3 A Semantic Web Primer
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Basic Ideas of RDF

Basic building block: object-attribute-value
triple
–
It is called a statement
–
Sentence about Billington is such a statement

RDF has been given a syntax in XML
–
This syntax inherits the benefits of XML
–
Other syntactic representations of RDF possible

Chapter 3 A Semantic Web Primer
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Basic Ideas of RDF (2)

The fundamental concepts of RDF are:
–
resources
–
properties
–
statements

Chapter 3 A Semantic Web Primer
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Resources

We can think of a resource as an object, a
“thing” we want to talk about
–
E.g. authors, books, publishers, places, people,
hotels

Every resource has a URI , a Universal
Resource Identifier

A URI can be
–
a URL (Web address) or
–
some other kind of unique identifier

Chapter 3 A Semantic Web Primer
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Properties

Properties are a special kind of resources

They describe relations between resources
–
E.g. “written by”, “age”, “title”, etc.

Properties are also identified by URIs

Advantages of using URIs:
–
Α global, worldwide, unique naming scheme
–
Reduces the homonym problem of distributed data
representation

Chapter 3 A Semantic Web Primer
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Statements

Statements assert the properties of
resources

A statement is an object-attribute-value triple
–
It consists of a resource, a property, and a value

Values can be resources or literals
–
Literals are atomic values (strings)

Chapter 3 A Semantic Web Primer
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Three Views of a Statement

A triple

A piece of a graph

A piece of XML code
Thus an RDF document can be viewed as:

A set of triples

A graph (semantic net)

An XML document

Chapter 3 A Semantic Web Primer
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Statements as Triples
(,
,
#David Billington)

The triple (x,P,y) can be considered as a