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Basic Ideas of RDF 2. RDF Serializations3. Basic Concepts of RDF Schema4. Τhe Language of RDF Schema5. Direct Semantics based on Inference RulesChapter 3 A Semantic Web Primer3Drawbacks of XMLXML is a universal metalanguage for defining markupIt provides a uniform framework for interchange of data and metadata between applicationsHowever, XML does not provide any means of talking about the semantics (meaning) of dataE.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 Primer4Nesting 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 Primer5Basic Ideas of RDFBasic building block: object-attribute-value triple–It is called a statement or a triple–Sentence about Billington is such a statement RDF has been given a syntax in XML–RDF/XML is the official syntax for representing RDF–Other syntactic representations of RDF possibleChapter 3 A Semantic Web Primer6Basic Ideas of RDF (2)The fundamental concepts of RDF are:–resources–properties–statementsChapter 3 A Semantic Web Primer7ResourcesWe can think of a resource as an object, a “thing” we want to talk about–E.g. authors, books, publishers, places, people, hotelsEvery resource has a URI, a Universal Resource Identifier A URI can be –a URL (Web address) or –some other kind of unique identifier –IRI: URI that supports UnicodeChapter 3 A Semantic Web Primer8PropertiesProperties are a special kind of resourcesThey 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 representationChapter 3 A Semantic Web Primer9StatementsStatements assert the properties of resourcesA statement is an object-attribute-value triple–It consists of a resource, a property, and a valueValues can be resources or literals –Literals are atomic values (strings)Examples of RDF Triples(http://www.cit.gu.edu.au#db, http://www.w3.org/1999/02/22-rdf-syntax-ns#type http://www.cit.gu.edu.au #lecturer)(http://www.cit.gu.edu.au#DBillington,http://www.example.com/predicates#hasName“David Billington”)Chapter 3 A Semantic Web Primer11Three Views of a StatementA tripleA part of a graphA piece of XML codeThus an RDF document can be viewed as:A set of triplesA graph An XML documentChapter 3 A Semantic Web Primer12Statements as Triples(http://www.cit.gu.edu.au#db, http://www.mydomain.org#site-owner, #David Billington)The triple (x,P,y) can be considered as a logical formula P(x,y)–Binary predicate P relates object x to object y –RDF offers only binary predicates (properties)–“The uncle problem”Chapter 3 A Semantic Web Primer13XML VocabulariesA directed graph with labeled nodes and arcs–from the resource (the subject of the statement) –to the value (the object of the statement)Known in AI as a semantic netThe value of a statement may be a resource–Ιt may be linked to other resourcesChapter 3 A Semantic Web Primer14A Set of Triples as a Semantic Nethttp://www.uni.edu/David_BillingtonSharing and Interchange with XMLWith XML, it was possible to represent the same semantics in multiple ways. It was not expressive enough on its own.A schema was necessary to structure the semantics embedded in the documentThe order of the tags is tied to the semantics of the documentHowever, many schemas might be possible for a given XML documentXML documents have a tree structureSharing and Interchange with RDFWith RDF, a separate schema definition to describe structure is not necessaryThis is because RDF triples can be described as a graphAny order of triple specification will result in the same graphThis makes it possible to combine arbitrary RDF graphs without caring about the order of combinationChapter 3 A Semantic Web Primer17Statements in XML SyntaxGraphs are a powerful tool for human understanding butThe Semantic Web vision requires machine-accessible and machine-processable representationsRDF can also be expressed as an XML document in RDF/XML formChapter 3 A Semantic Web Primer18Data TypesData types are used in programming languages to allow interpretationIn RDF, typed literals are used, if necessary(#David Billington,http://www.mydomain.org/age,“27”^^http://www.w3.org/2001/XMLSchema#integer)Chapter 3 A Semantic Web Primer19Data Types (2)^^-notation indicates the type of a literal In practice, the most widely used data typing scheme will be the one by XML Schema –But the use of any externally defined data typing scheme is allowed in RDF documentsXML Schema predefines a large range of data types–E.g. Booleans, integers, floating-point numbers, times, dates, etc.Chapter 3 A Semantic Web Primer20Binary PredicatesRDF uses only binary properties–This is a restriction because often we use predicates with more than 2 arguments–But binary predicates can simulate theseExample: referee(X,Y,Z) –X is the referee in a chess game between players Y and ZChapter 3 A Semantic Web Primer21Binary Predicates (2)We introduce:–a new
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