Lecture notes in Semantic Web

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Published Date:11-07-2017
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Semantic Web Basics (cont.) CS 431 – March 26, 2008 Carl Lagoze – Cornell University Acknowledgements for various slides and ideas •  Ian Horrocks (Manchester U.K.) •  Eric Miller (W3C) •  Dieter Fensel (Berlin) •  Volker Haarslev (Montreal) We started with the Presentation Web Atom/RSS gave us data extraction from the Web Simple Compound Document Aggregator But we want to do more – Beyond just web pages Events Events People We want to do more – Compound/Complex Relationships Simple Compound Document Motivating the problem: Integrating Web Resources in new ways Standards/mechanisms for doing this •  Stuff we’ve learned so far –  URIs – keys for unique identity and joining distributed information –  XML – Markup for serialization of knowledge bases –  Namespaces – URIs for vocabulary terms •  Stuff we’ll learn from here –  RDF – basic model for representing knowledge via binary relationships –  Ontologies – definitions of vocabulary terms and their relationships –  OWL – RDF-based model for expressing Ontologies –  Description logic – Formal way to represent ontologies and reason with them Assertions are statements •  Resource1 “is about” Resource2 •  Resource1 “annotates” Resource2 •  Resource1 “illustrates” Resource2 •  Organization1 “owns” Resource2 •  Person1 “recommends” Resource2 •  RDF is a model for making assertions –  Subject  Predicate  Object RDF Data Model •  Directed Graph expressing typed binary relations between typed resources •  Relations are: –  P(S,O) or (:s :p :o) •  Primitives –  resource –  property –  literal –  statement •  Other constructs –  container –  reification –  collection •  URI’s for everything except literals –  “bnodes” are a special case, but more about that later •  Common serialization is RDF/XML Why URIs •  Purpose of RDF is integrating information from multiple sources –  Existing web –  Introduced entities (people, organizations, taxonomies) •  URI’s form basis of joins of graph •  Instance data combines into larger graphs •  Inferences can be made based on: –  RDF primitives –  Ontology definitions •  RDFs •  OWL RDF Model Primitives Property Resource Value Resource Statement RDF Model Example 2 RDF/XML Syntax Example 2 Typed Literals Beyond binary relations •  Note mapping of RDF statements to binary relations that could be stored in a database: –  (:s :p :o) maps to P(S,O) – e.g., Title(R, “War & Peace”) •  But the world is more complex and statements are arbitrary n-tuples –  Carl Lagoze has his office at 301 College Ave., Ithaca, NY 14850 –  (“Carl Lagoze” “hasOffice” “301 College Ave, Ithaca, NY 14850”) –  (“Carl Lagoze” “address” “301 College Ave” “Ithaca” “NY” “14850”) Expressing n-ary relations with blank nodes URI 1 address “blank node” (think of as local variable) street city state zip “NY” “301 College Ave” “Ithaca” “14850” Another n-ary relation example RDF Containers •  Permit the aggregation of several values for a property •  Express multiple aggregation semantics –  unordered –  sequential or priority order –  alternative RDF Containers •  Bag –  unordered grouping •  Sequence –  ordered grouping •  Alternatives –  alternate values •  need to choose –  at least one value –  first value is default or preferred value

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