Lecture the semantic_web_part_1

Co-funded by the European Union

Semantic CMS Community

The Semantic Web – Part 1

The Vision and Basic Technologies of the Semantic Web.

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Introduction of Content Management

Foundations of Semantic Web Technologies

Storing and Accessing Semantic Data

Knowledge Interaction and Presentation

Knowledge Representation and Reasoning

Semantic Lifting

Designing Interactive Ubiquitous IS

Requirements Engineering for Semantic CMS

Designing Semantic CMS

Semantifying your CMS

Part I: Foundations

Part II: Semantic Content Management

Part III: Methodologies

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What is this Lecture about? Semantic web technologies as a possible solution for

improving the „state of play“ in content management by making semantics expressable in machine-readable way

The Semantic Web The vision behind the semantic web Underlying technologies of the Semantic Web


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Introduction of Content Management

Foundations of Semantic Web Technologies

Part I: Foundations

(2) (1)

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Web evolution

Slide by Nova Spivack, Radar Networks

Web 1.0 Web 2.0

Web 3.0 Web 4.0

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The Semantic Web

The vision of the Semantic Web has been originally proposed by Tim Berners-Lee

“The Semantic Web is not a separate Web but an extension of the current one, in which information is given well-defined meaning, better enabling computers and people to work in cooperation.” [The Semantic Web, 2001]

Standardized specification techniques for the semantic annotation of content (RDF, OWL, ...)


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Semantic Web Stack W3C provides standardized

specifications for Semantic Web technologies

Semantic Web Layer Cake as a conceptual architecture describes an hierarchy of languages

Each layer exploits and uses capabilities of the layers below


Semantic Web Layer Cake, Image source: http://www.w3.org/2007/03/layerCake.svg

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Unique Identification of Resources

“...more fundamental than either HTTP or HTML are URIs, which are simple text strings that refer to Internet resources -- documents, resources, people, and indirectly to anything. URIs are the glue that binds the Web together. IRIs extend and strengthen the glue, by allowing people to identify Web resources

in their own language.”

In a “Web of Data” the unique

identification of entities is required


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How to identify resources?

URI – Uniform Resource Identifier [RFC 3986] “A Uniform Resource Identifier (URI) is a compact

sequence of characters that identifies an abstract or physical resource.”

A URI consists of five parts: scheme, authority, path, query and fragment

URI = scheme ":" authority "/" path [ "?" query ] [ "#" fragment ]

Example:


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scheme authority path query fragment

http://[email protected]:8042/over/there?name=ferret#nose

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How to identify resources?

IRI – Internationalized Resource Identifier [RFC 3987] “IRIs are defined similarly to URIs in [RFC3986], but the

class of unreserved characters is extended by adding the characters of the UCS (Universal Character Set, [ISO10646]) ..”

Extends the character set used by URIs and thereby allows the consideration of language specific syntaxes (e.g. Japanese, Chinese, ...)

Example Logical Hebrew http://ab.גדהוזח.ij/kl/mn/op.html ASCII notation http://ab.CDEFGH.ij/kl/mn/op.html


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Structuring Data

XML – Extensible Markup Language XML can be used to specify the syntactical structure of

documents or complex data objects in a machine-readable form


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<car> <brand>Jaguar</brand> <model>XF</model> <dimensions> <length>4961</length> <width>2070</width> </dimensions> <engine>5.0 L V8 Petrol</engine></car>

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Why isn´t XML enough?

XML tags are only defining the structure of a document For a machine the tags have no semantics!


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<car> <brand>Jaguar</brand> ...</car>

<cat> <brand>Jaguar</brand> ...</cat>

The difference in meaning is only recognized by a human.

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Why isn´t XML enough?

The relation among the different tags is not expressed explicitly


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<car>

<brand>Jaguar</brand> ...

</car>

The relation between “car”, “brand” and “Jaguar” is only recognizable

by a human.

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What do we need?

We want to express the statement: “The brand of the car is Jaguar.”

We need ... ...a way to address the concrete resource car. ... to express the property brand of the resource car. ... to define the property value Jaguar for the property

brand.


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Resource Description Framework (RDF)

“The Resource Description Framework (RDF) identifies things using Web identifiers (URIs), and describes resources with properties and property values.”

A Resource is an object that can be identified by an URI, e.g. “http://example.org/Car”.

A Property describes an aspect of a resource, e.g. “http://example.org/Brand”. The property is also identified by an URI.

The Property value assigns a concrete value to a property, e.g. “Jaguar” or ““http://example.org/Jaguar”.

Copyright IKS Consortiumhttp://www.w3schools.com/rdf/

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RDF Statements

RDF statements consist of subject (resource), predicate (property) and object (property value)

Subjects (except Blank Nodes) and Predicates are always defined by URIs

Objects can be defined by URIs and literals


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SubjectObject (URI)

Predicate

Object (literal)

Predicate

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RDF Statements - Example

Exemplary statements: “The brand of the car is Jaguar.” “The model of the car is XF.”


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http://example.org/Carhttp://example.org/

Jaguar

http://example.org/rel/Brand

XF

http://example.org/rel/Model

Subject Predicate Object

Object

Predicate

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Resource Description Framework (RDF)

“The Resource Description Framework (RDF) is a language for representing information about resources...” [RDF Primer]

• W3C Standard (http://www.w3.org/RDF)

RDF provides a graph-based data model for representing metadata for describing the semantics of

information in a machine-accessible way


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RDF Serialization Formats

RDF/XML N3 N-Triples TRiG TRiX Turtle JSON JSON-LD RDFa


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RDF/XML

RDF/XML is the most common serialization format for RDF statements

Example:


http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:rel="http://example.org/rel/">

<rdf:Description rdf:about="http://example.org/Car"> <rel:Brand rdf:resource="http://example.org/Jaguar"/> </rdf:Description> <rdf:Description rdf:about="http://example.org/Car"> <rel:Model>XF</rel:Model> </rdf:Description></rdf:RDF>

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Root Element and Namespaces


<rdf:RDFxmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:rel="http://example.org/rel/">

....

</rdf:RDF>

<rdf:RDF> Root element of RDF documents

xmlns:rdf Specifies that elements with the “rdf” prefix are using the namespace “..”

xmlns:abc Specifies that elements with the “abc” prefix are using the namespace “..”

Root element of the RDF/XML document.

Defined namespaces

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Description Element


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<rdf:Description rdf:about="http://example.org/Car"> ... </rdf:Description> ...

</rdf:RDF>

<rdf:Description> Introduces node element (subject).

<rdf:about> Defines the URI reference for the node element.

Node element

Assigns a resource to the node element.

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Objects defined by Resources

Assigning resources to objects (property values) in RDF/XML


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<rdf:Description rdf:about="http://example.org/Car"> <rel:Brand rdf:resource="http://example.org/Jaguar"/> </rdf:Description>

</rdf:RDF>

http://example.org/Car http://example.org/Jaguar

http://example.org/rel/Brand

The object is defined by a resource.

Defines predicate “Brand” in namespace “rel”.

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Objects defined by Literals

Definition of objects (properties) with constant values (literals)


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<rdf:Description rdf:about="http://example.org/Car"> <rel:Model>XF</rel:Model> </rdf:Description>

</rdf:RDF>http://example.org/Car

XF

http://example.org/rel/Model

The object is defined by a literal.

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RDF Container Elements

“A container is a resource that contains things. The contained things are called members. The members of a container may be resources (including blank nodes) or literals.”

Container provide the ability to describe groups of things, e.g. several authors of a book

RDF defines three types of containers: Bag Sequence Alternatives


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http://www.w3.org/TR/2004/REC-rdf-primer-20040210/

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Container Element - Bag

“A Bag (a resource having type rdf:Bag) represents a group of resources or literals, possibly including duplicate members, where there is no significance in the order of the members.”

Example: Describe equipment features of the car.



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http://example.org/Car

http://www.w3.org/1999/02/22-rdf-syntax-

ns#Bag

http://www.w3.org/1999/02/22-rdf-syntax-ns#type

http://example.org/AC

http://example.org/CarRadio

http://example.org/SeatHeating

http://www.w3.org/1999/02/22-rdf-syntax-ns#_1



http://example.org/features#equipment

Blank Node

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<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:e="http://example.org/features#">

<rdf:Description rdf:about="http://example.org/Car"><e:equipment> <rdf:Bag>

<rdf:li rdf:resource="http://example.org/AC"/><rdf:li rdf:resource="http://example.org/CarRadio"/><rdf:li rdf:resource="http://example.org/SeatHeating"/>

</rdf:Bag> </e:equipment>

</rdf:Description>

</rdf:RDF>

<x:xyz> Defines predicate “xyz” in namespace “x”.

<rdf:Bag> Defines a container of type “Bag”.

<rdf:li> Defines members of the Bag container.

The subject “Car” is described by the

predicate “equipment”.

The different members of the Bag container

are listed.The object is a

container of type “Bag”

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Container Element - Sequence

“A Sequence or Seq (a resource having type rdf:Seq) represents a group of resources or literals, possibly including duplicate members, where the order of the members is significant.”

Example: Describe a list of previous owners of the car in

chronological order.



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http://example.org/Carhttp://www.w3.org/

1999/02/22-rdf-syntax-ns#Seq


http://example.org/JohnDoe

http://example.org/RickMiller

http://example.org/SarahHouse




http://example.org/owner#preOwners

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<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:p="http://example.org/owner#">

<rdf:Description rdf:about="http://example.org/Car"><p:preOwners> <rdf:Seq>

<rdf:li rdf:resource="http://example.org/JohnDoe"/><rdf:li rdf:resource="http://example.org/RickMiller"/><rdf:li rdf:resource="http://example.org/SarahHouse"/>

</rdf:Seq> </p:preOwners>

</rdf:Description>

</rdf:RDF>


<rdf:Seq> Defines a container of type “Sequence”.

<rdf:li> Defines members of Sequence container.


predicate “preOwners”.

The different members of the Sequence

container are listed.The object is a

container of type “Sequence”.

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Container Element - Alternative

“An Alternative or Alt (a resource having type rdf:Alt) represents a group of resources or literals that are alternatives (typically for a single value of a property).”

Example: Describe a list of possible colors of the car.



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ns#Alt


http://example.org/Black

http://example.org/White

http://example.org/Green




http://example.org/exterior#colors

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<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:c="http://example.org/exterior#"

<rdf:Description rdf:about="http://example.org/Car"><c:colors> <rdf:Alt>

<rdf:li rdf:resource="http://example.org/Black"/><rdf:li rdf:resource="http://example.org/White"/><rdf:li rdf:resource="http://example.org/Green"/>

</rdf:Alt> </c:colors>

</rdf:Description>

</rdf:RDF>


<rdf:Alt> Defines a container of type “Alternative”.

<rdf:li> Defines members of Alternative container.


predicate “colors”.

The different members of the Alternative

container are listed.The object is a

container of type “Alternative”.

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RDF Collections

A RDF Container can be used to define groups, but lacks the ability to “close” them. That means to define that "these are all the members of the container".

RDF Collections provide the ability to describe groups, that contain only the specified resources.

“An RDF collection is a group of things represented as a list structure in the RDF graph.”

Example: All equipment features of a car.



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RDF Collection


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ns#nil

http://www.w3.org/1999/02/22-rdf-syntax-ns#rest

http://example.org/AC

http://example.org/CarRadio

http://example.org/SeatHeating

http://www.w3.org/1999/02/22-rdf-syntax-ns#_first

http://example.org/features#equipment





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RDF Collection


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<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"xmlns:e="http://example.org/features#">

<rdf:Description rdf:about="http://example.org/Car"><e:equipment rdf:parseType="Collection"> <rdf:Description rdf:about="http://example.org/AC"/> <rdf:Description rdf:about="http://example.org/CarRadio"/> <rdf:Description rdf:about="http://example.org/SeatHeating"/> </e:equipment>

</rdf:Description>

</rdf:RDF>

<x:xyz rdf:parseType=“Collection”>

Defines that the predicate “xyz” is described by a collection.

The predicate “equipment” is described

by a Collection.

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Validation Tool


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http://www.w3.org/RDF/Validator/

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Semantic Layer Web Cake


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Unique identification of resources

A format for specifying structured data in a machine-readable form

A model for describing resources with properties

and property values.

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RDF

RDF provides a model for describing resources with properties and property values.


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@prefix ex: <http://www.example.org/>.

ex:Car1 ex:Brandex:Jaguarex:Car1 ex:Colour “Black”ex:Car2 ex:Brandex:Jaguarex:Car2 ex:Colour “White”ex:Car3 ex:Brand ex:VWex:Car3 ex:Colour “Black”

How do I get all

black Jaguars?

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SPARQL

SPARQL Protocol and RDF Query Language W3C Recommendation since 2008

SPARQL provides a standard for querying information, that is specified in RDF

SPARQL consists of three specifications Query language Query results XML format Data access protocol


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Exemplary SPARQL Query

“Return the models and prices for all cars of brand ‘Jaguar’ ”

SPARQL Query:

Exemplary Result:


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PREFIX ex: <http://example.org/> SELECT ?model ?priceWHERE { ?car ex:Brand ex:Jaguar . ?car ex:Model ?model . ?car ex:Price ?price . }

Model Price

“XJ” “79.750,00”

“XF” “44.900,00”

Declares namespaces for abbreviated resources

identifiers.

Identifies the variables to appear in the query

results.

Provides the basic graph pattern to match against

the data graph.

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Basic Graph Patterns (BGP)

A SPARQL query contains a set of triple patterns called a basic graph pattern

Triple patterns are like RDF triples except that each of the subject, predicate and object may be a variable.

A basic graph pattern matches a subgraph of the RDF data when RDF terms from that subgraph may be substituted for the variables and the result is a RDF graph equivalent to the subgraph.


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WHERE { ?car ex:Brand ex:Jaguar . ?car ex:Model ?model . ?car ex:Price ?price . }

Variables are defined by “?varname” or “$varname”

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Group Graph Patterns

“For a Group Graph Pattern, a set of graph patterns must all match”

In a SPARQL query string, a group graph pattern is delimited with braces: {...}


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PREFIX ex: <http://example.org/> SELECT ?model ?priceWHERE { { ?car ex:Brand ex:Jaguar . ?car ex:Model ?model . }

?car ex:Price ?price . }

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Optional Pattern Matching

Basic graph patterns allow applications to make queries where the entire query pattern must match.

It is useful to be able to have queries that allow information to be added to the solution where the information is available, but do not reject the solution because some part of the query pattern does not match.

If the optional part does not match, it creates no bindings but does not eliminate the solution.


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http://www.w3.org/TR/rdf-sparql-query

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Optional Pattern Matching

Example:

Result


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PREFIX ex: <http://example.org/> SELECT ?model ?priceWHERE { ?car ex:Brand ex:Jaguar . ?car ex:Model ?model . OPTIONAL { ?car ex:Price ?price . } }

Model Price

“XJ” “79.750,00”

“XF” “44.900,00”

“S-Type”

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Scope of Filters

“A constraint, expressed by the keyword FILTER, is a restriction on solutions over the whole group in which the filter appears.”

Result:


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PREFIX ex: <http://example.org/> SELECT ?model ?priceWHERE { ?car ex:Brand ex:Jaguar . ?car ex:Model ?model . ?car ex:Price ?price . FILTER (?price < 50.000,00)}

Model Price

“XF” “44.900,00”

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Unary Operators (1/2)


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Operator Type(A) Result Type

BOUND(A) variable xsd:boolean

IsURI(A) RDF term xsd:boolean

isBLANK(A) RDF term xsd:boolean

isLITERAL(A) RDF term xsd:boolean

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Unary Operators (2/2)


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Operator Type(A) Result Type

STR(A) literal simple literal

STR(A) URI / IRI simple literal

LANG(A) literal simple literal

DATATYPE(A) typed literal URI / IRI

DATATYPE(A) simple literal URI / IRI

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Binary Operators (1/2)


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Operator Type(A) Type(B) Result Type

A = B

numericsimple literal

xsd:stringxsd:boolean

xsd:dateTime

numericsimple literal

xsd:stringxsd:boolean

xsd:dateTime

xsd:boolean

A != B

A < B

A > B

A <= B

A >= B

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Binary Operators (2/2)

Filter can be combined by “&&” or “||”


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Operator Type(A) Type(B) Result Type

sameTERM(A,B) RDF term RDF term xsd:boolean

langMATCHES(A,B) simple literal simple literal xsd:boolean

REGEX (A,B) simple literal simple literal xsd:boolean

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Alternative Graph Patterns

“SPARQL provides a means of combining graph patterns so that one of several alternative graph patterns may match. If more than one of the alternatives matches, all the possible pattern solutions are found.”

Pattern alternatives are syntactically specified with the UNION keyword.


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PREFIX ex: <http://example.org/> SELECT ?officeWHERE { ?x rdf:type ex:CarManufacturer . { ?x ex:Headquarter ?office . } UNION { ?x ex:Office ?office . } }

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Solution Sequences and Modifiers

“Query patterns generate an unordered collection of solutions. These solutions are then treated as a sequence (a solution sequence), initially in no specific order; any sequence modifiers are then applied to create another sequence.”

There are several use cases in which the results shall be presented in a defined order, e.g. alphabetical order

Shall duplicates be presented or deleted?


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Order By

“Following the ORDER BY clause is a sequence of order comparators, composed of an expression and an optional order modifier (either ASC() or DESC()). Each ordering comparator is either ascending (indicated by the ASC() modifier or by no modifier) or descending (indicated by the DESC() modifier).”


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PREFIX ex: <http://example.org/>

SELECT ?constructionYear ?priceWHERE { ?car ex:Brand ex:Jaguar .

?car ex:ConstructionYear ?constructionYear . ?car ex:Price ?price . }

ORDER BY DESC(?constructionYear) ?price .

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Some more Solution Sequence Modifiers

LIMIT “The LIMIT clause puts an upper bound on the number of

solutions returned. If the number of actual solutions is greater than the limit, then at most the limit number of solutions will be returned.”

OFFSET “OFFSET causes the solutions generated to start after the

specified number of solutions. An OFFSET of zero has no effect.”

SELECT DISTINCT “The DISTINCT solution modifier eliminates duplicate solutions.

Duplicates are eliminated before either limit or offset is applied.”


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Semantic Web Layer Cake


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Unique identification of resources

A format for specifying structured data in a machine-readable form

A model for describing resources with properties

and property values.

A language for querying information specified in

RDF.

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Lessons Learned

Now you should know ... ... the general idea of the Semantic Web and the

differences to the World Wide Web. ... the underlying technologies of the Semantic Web. ... how resources can be identified and specified. .. the importance of XML for expressing data in a machine-

readable way. ... the concept of the RDF and the way it is expressed

(e.g.: RDF/XML). ... how SPARQL can used to query data specified in RDF.


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References and Additional Material

Introduction to RDF http://www.w3schools.com/rdf/rdf_intro.asp

RDF Syntax Specification http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-

20040210 RDF Validation Tool

http://www.w3.org/RDF/Validator/


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