CH-4 Ontologies, Querying and Data Integration

Introduction to RDF(S) RDF stands for Resource Description Framework. RDF is a standard for describing Web resources. RDF can be used to describe title, author, content, and copyright information of web pages.

RDF - Examples of Use Describing properties for shopping items, such as price and availability Describing time schedules for web events Describing information about web pages (content, author, created and modified date) Describing content and rating for web pictures Describing content for search engines Describing electronic libraries

RDF Example Subject: Nick Predicate: hasColleague Object: Matthew RDF uses URIs - Unique Resource Identifiers to identify resources.

Linking Statements  The subject of one statement may be the object of another statement.  A set of linked statements (triples) forms an RDF Graph.

An RDF Graph Example

RDF Schema (RDFS) - The RDF Vocabulary Description Language RDF Schema ‘semantically extends’ RDF to enable us to talk about classes of resources, and the properties that will be used with them. It does this by giving special meaning to certain rdf properties and resources. RDF Schema provides the means to describe application specific RDF vocabularies. A set of URI Refs is known as a vocabulary.

Describing Classes with RDFS To describe classes we can use built in RDF Schema resources: rdfs:Class rdfs:subClassOf These are used in conjunction with the rdf:type property.

Describing Properties with RDF(S) RDF Schema allows us to describe properties. (Properties are instances of the class rdf:Property !) We can specify a domain using rdfs:domain. We can specify a range using rdfs:range.

RDF(S) Summary RDF (“Resource Description Framework”), which is the standard for encoding metadata and other knowledge on the Semantic Web. RDF - The Resource Description Framework allows us to describe resources by specifying their properties and property values. RDF Statements are triples of the form [Subject, Predicate, Object] A set of RDF triples forms an RDF Graph. RDF Schema semantically extends RDF by providing a means to describe RDF Vocabularies. RDF is an abstract model, a way to break down knowledge into discrete pieces, and while it is most popularly known for its RDF/XML syntax, RDF can be stored in a variety of formats.

RDF EXAMPLE <rdf:RDF xmlns:rdf=" rdf-syntax-ns#" xmlns:dc=" World Wide Web Consortium

PARSED OUTPUT

OWL What is Ontology? What is OWL? Why OWL?

What is Ontology? Ontology is about the exact description of things and their relationships. For the web, ontology is about the exact description of web information and relationships between web information.

What is OWL? OWL stands for Web Ontology Language OWL is built on top of RDF OWL is for processing information on the web OWL was designed to be interpreted by computers OWL was not designed for being read by people OWL is written in XML OWL has three sublanguages OWL is a W3C standard

Why OWL? OWL is a part of the "Semantic Web Vision" - a future where: Web information has exact meaning Web information can be processed by computers Computers can integrate information from the web

The “Layer Cake”

Components of an OWL Ontology Individuals Properties Classes

The Three Species of OWL OWL-Full - No restrictions on how/where language constructs can be used. The union of OWL and RDF(S). OWL-Full is not decidable. OWL-DL - Restricted version of OWL-Full. Corresponds to a description logic. Certain restrictions on how/where language constructs can be used in order to guarantee decidability. OWL-Lite - A subset of OWL-DL. The simplest and easiest to implement of the three species.

OWL Summary OWL is the latest standard in ontology languages. It is layered on top of RDF and RDFS, and has a rich set of constructs. There are three species of OWL: OWL-Lite, OWL-DL and OWL-Full. We can perform automated reasoning over ontologies that are written in OWL-Lite and OWL-DL.