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  • Exercise 1

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  • 1. + RDFS A little semantics goes a long way Mariano Rodriguez-Muro, Free University of Bozen-Bolzano
  • 2. + Disclaimer  License   A few examples from these slides has been taken from   This work is licensed under a Creative Commons Attribution-Share Alike 3.0 License (http://creativecommons.org/licenses/by-sa/3.0/) Semantic Web for the working Ontologist. Chapter 6. Some of the slides on the use of taxonomies are based on:  http://info.earley.com/webinar-replay-business-value-taxonomy-aug2012
  • 3. + Reading material  Semantic Web for the working Ontologist. Chapter 6 http://proquest.safaribooksonline.com/book/-/9780123859655
  • 4. + Motivation  Ontologies  RDFS ontologies  Overview  Annotations
  • 5. + Motivation
  • 6. + Motivation  Motivations for semantic technology  Making the web machine understandable  Expressing knowledge   However, points 2 and 3 are not possible with the technologies seen so far  RDF doesn’t define vocabularies, and  Different datasets may use different URI’s to represent the same kind of data Reasoning with knowledge
  • 7. + Motivation  Agreement in RDF graphs concerns only the data model and the use of URI as identifiers  No semantics!
  • 8. + Ontologies and Ontology Languages
  • 9. + What is an ontology language?  Specification of valid “axioms”  Specifications of vocabularies with “predefined” meaning in axioms  Informal: Topic Maps, UML diagrams  Formal examples: Predicate Logic, First Order Logic  Semantic Web examples:    RDFS SWRL, OWL  Different languages have different expressive power  Axioms allow to produce “inferences”  The more expressive power, more complex and costly the inferences
  • 10. + What is an ontology?  Collections of “axioms”  Describe the meaning of the vocabulary of a domain (e.g., an area of expertise)  Expressed in an Ontology Language  Valuable on their own as knowledge repositories  In combination with data valuable to implement complex behavior with little or no coding
  • 11. + Example: Schema.org  Schema.org IS a simple ontology  Organizes terms in hierarchies with predefined meaning  The language is a variation of RDFS
  • 12. + RDFS Introduction by example
  • 13. + RDFS  W3C standard for an ontology language  RDFS introduces resources (URIs) with a predefined meaning  Inference engines that support RDFS allow to take that meaning into account  RDFS inferences extend the RDF graph by means of inference and hence, affect query answering  RDFS is very simple compared to SWRL or OWL, however, it is very useful in many context, allowing for increased productivity, easy data integration and interesting AI applications
  • 14. + Building blocks  New namespace rdfs: <http://www.w3.org/2000/01/rdf-schema#>  New categories: Commonly, Class names are nouns  Classes, resources that share something in common, allow us to group things together. For example, Employee, Company. Resources that identify classes have rdf:type rdfs:Class  Instances, resources that are “members” of a class
  • 15. + Building blocks Resources can belong to multiple classes
  • 16. + Building blocks (cont.)  Properties: Resources used as a predicate in statements Commonly, Property names are multiple words, expressing direction and in camel-casing
  • 17. + RDFS Ontologies  RDFS Axioms  Are RDF triples!  RDFS ontology is an RDF graph!  An RDF graph may have a subgraph expressed in RDFS  We call the RDFS axioms/triples the Tbox of the ontology (terminological information, predefined meaning)  The rest is the Abox of the ontology (plain data, no predefined meaning)
  • 18. + Type propagation  RDFS vocabulary: rdfs:subClassOf  Key notions  sub class (on the left)  super class (on the right)  Intuitive meaning, if :mariano is an instance of subclass it is also an instance of superclass  Formal meaning: subsets  Inference: type propagation Similar to inheritance in Object Oriented formalisms
  • 19. + Type propagation  RDFS vocabulary: rdfs:subClassOf  Key notions  sub class (on the left)  super class (on the right)  Intuitive meaning, if :mariano is an instance of subclass it is also an instance of superclass  Formal meaning: subsets  Inference: type propagation Similar to inheritance in Object Oriented formalisms
  • 20. + Relation propagation  RDFS vocabulary: rdfs:subPropertyOf  Key notions  sub property(on the left)  super property(on the right)  Intuitive meaning, if (x,y) are connected with subproperty they are also connected with superproperty  Formal meaning: subsets (of binary tuples)  Inference: relationship propagation
  • 21. + Relation propagation  RDFS vocabulary: rdfs:subPropertyOf  Key notions  sub property(on the left)  super property(on the right)  Intuitive meaning, if (x,y) are connected with subproperty they are also connected with superproperty  Formal meaning: subsets (of binary tuples)  Inference: relationship propagation
  • 22. + Types by usage  RDFS vocabulary: rdfs:domain, rdfs:range  Key notions  domain of a triple: the subject  range of a triple: the object  :p rdfs:domain :C > the domain of any triple where :p is the predicate is an instance of :C (similar for rdfs:range)  Formal meaning: if (x,y) in P, then x in :C  Inference: type assignment by property usage
  • 23. + Types by usage  RDFS vocabulary: rdfs:domain, rdfs:range  Key notions  domain of a triple: the subject  range of a triple: the object  :p rdfs:domain :C > the domain of any triple where :p is the predicate is an instance of :C (similar for rdfs:range)  Formal meaning: if (x,y) in P, then x in :C  Inference: type assignment by property usage
  • 24. + Interactions  All inferences interact to allow complex behavior
  • 25. + Interactions  All inferences interact to allow complex behavior
  • 26. + Set intersection  Proper set intersection is not possible in RDFS  However, expressing necessary membership to multiple classes is possible, i.e., A subset B AND C A rdfs:subClassOf B A rdfs:subClassOf C consider x rdf:type A
  • 27. + Set intersection  Proper set intersection is not possible in RDFS  However, expressing necessary membership to multiple classes is possible, i.e., A subset B AND C A rdfs:subClassOf B A rdfs:subClassOf C consider x rdf:type A
  • 28. + Set intersection  Proper set intersection is not possible in RDFS  However, expressing necessary membership to multiple classes is possible, i.e., A subset B AND C A rdfs:subClassOf B A rdfs:subClassOf C consider x rdf:type A One direction only!
  • 29. + Set intersection  Similar for roles
  • 30. + Set intersection  Similar for roles
  • 31. + Set union  Proper set union is not possible in RDFS  However, A OR B subsetOf C B rdfs:subClassOf A C rdfs:subClassOf A consider x rdf:type B or x rdf:type C
  • 32. + Set union  Proper set union is not possible in RDFS  However, A OR B subsetOf C B rdfs:subClassOf A C rdfs:subClassOf A consider x rdf:type B or x rdf:type C
  • 33. + Set union  For roles. Aligning to a global vocabulary
  • 34. + Set union  For roles. Aligning to a global vocabulary
  • 35. + Equivalence  Merging vocabularies  To account for same use of different terms (classes or properties)  For classes or proeperties
  • 36. + Equivalence  Merging vocabularies  To account for same use of different terms (classes or properties)  For classes or proeperties
  • 37. + Last notes on RDFS axioms  Main new vocabulary:  rdfs:subClassOf  rdfs:subPropertyOf  rdfs:domain  rdfs:range  Different from CONSTRAINTS, missing triples are NOT a violation  Allow to infer new information  Allows to implement system behavior!
  • 38. + Open lists revisited  RDFS also facilitates access to Lists  Elements of lists are a possibly infinite set of elements of the form rdf:_1, rdf:_2, etc RDFS facilitates this by enforcing that: if x rdfs:_1 y then x rdfs:member b  Access difficult in practice
  • 39. + Open lists revisited  RDFS also facilitates access to Lists  Elements of lists are a possibly infinite set of elements of the form rdf:_1, rdf:_2, etc  Access difficult in practice RDFS facilitates this by enforcing that: if x rdfs:_1 y then x rdfs:member b More detail on this on the lecture about RDFS semantics
  • 40. + Axiomatic triples  RDFS enforces certain facts to be always true  These facts are statements (triples)  Referred as Axiomatic triples  Listed in http://www.w3.org/TR/rdf-mt/ More detail on this on the lecture about RDFS semantics
  • 41. + RDFS Semantic Conditions  Every resource x x rdf:type rdfs:Resource  Every literal x x rdf:type rdfs:Literal  … etc More detail on this on the lecture about RDFS semantics
  • 42. + Last notes on RDFS axioms  Main new vocabulary (not the only one):  rdfs:subClassOf  rdfs:subPropertyOf  rdfs:domain  rdfs:range  Different from CONSTRAINTS, missing triples are NOT a violation  Allow to infer new information  Allows to implement system behavior!
  • 43. + Hands on examples From Semantic Web for the Working Ontologist
  • 44. + Automatic classification of employees (part 1)  Transform into an RDF representation  Automatically catalog objects as Employees, and as Active employees, Suspended employees and Ex-employees using a minimal set of “axioms” <ID> Project Assignment Absent Until Termination Date 22 24 - - 34 24 Dec 23, 2012 - 73 - - Jun 4, 2010 Employee table. Primary key: 10 Active employees are assigned to projects
  • 45. + Automatic classification of employees (part 2)  Transform into an RDF representation  Automatically catalog objects as Employees as managers <ID> Project Name <Manager> 24 Project-x 34 25 Project Mayhem 22 Project table. Primary key: ID Foreign key <Manager> to Employee table
  • 46. + Align vocabularies • Align corresponding properties using RDFS • Align with FOAF vocabulary (when possible) using RDFS (use foaf:name, foaf:homepage)
  • 47. + Annotations
  • 48. + Annotations  URI’s are not readable  Readable information (comments, names, etc.) can be stored using properties, but  Property names are not standard, however, we could like some standard names for “human oriented information”  RDFS defines:  rdfs:label A readable name for a resource  rdfs:comment Human focused comments These are properties So, subPropertyOf can be used with them
  • 49. + Redirection  Redirecting to location of documents (RDF) with additional information about a subject  No formal semantics  RDFS provides:  rdfs:seeAlso. Additional information  rdfs:definedBy. Authority information, primary source. Recall the semantic web idea, linked databases These are properties So, subPropertyOf can be used with them