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Introduction to the
Semantic Web

by zaki

2012-01-26

----
== Introduction

What?
     A collection of technologies for organizing data and
     their relationships with other data in a machine-readable
     format.
+

   It's all about metadata.

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Relational model? SQL?

+
- Relational DBs use tables
    rows and columns, strict set of data per entity,
    need ORMs to help think about extensions
+
- Semantic Web tech uses graphs [1]
    allows for looser data
      - missing values,
      - multiple relationships per entity

[1]: a simplification, actually a hypergraph
     <http://www.appzdata.com/fyd/2011/08/28/rdf-is-not-a-directed-labeled-graph/>

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graph -> explore the data

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== RDF: Resource Description Framework

model for representing relatioships

based on sets of triples

   [subject] [predicate] [object]

+ e.g.

   [earth_sky] [has_color] [color_blue]

+ except RDF uses URIs as identifiers.

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RDF is not a file format

multiple serializations, e.g.:

- XML (cumbersome to write by hand)
- Turtle & N3 (newer)

Turtle is the simplest

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It looks like this:

 ----------------------------------------------------------
 | @prefix dc: <http://purl.org/dc/elements/1.1/> .       |
 |                                                        |
 | <http://cougarcs.com/> dc:title "CougarCS".            |
 | <http://www.uh.edu/> dc:title "University of Houston". |
 ----------------------------------------------------------

----
 ----------------------------------------------------------
 | @prefix dc: <http://purl.org/dc/elements/1.1/> .       |
 |                                                        |
 | <http://cougarcs.com/> dc:title "CougarCS".            |
 | <http://www.uh.edu/> dc:title "University of Houston". |
 ----------------------------------------------------------

The @prefix part refers to a namespace that is expanded.

 ------------------------------------------------------------------
 | <http://cougarcs.com/> <http://purl.org/dc/elements/1.1/title> |
 |      "CougarCS".                                               |
 ------------------------------------------------------------------
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As a graph this would be:

 -------------------------------------------------------
 | --------------------------  dc:title   ------------ |
 | | <http://cougarcs.com/> | ----------> | CougarCS | |
 | --------------------------             ------------ |
 -------------------------------------------------------

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<http://purl.org/dc/elements/1.1/> 
  - URI for the Dublin Core metadata standard
  - defines a vocabulary for describing works

    title, creator, subject, description, publisher,
    contributor, date, type, format, identifier, source,
    language, relation, coverage, rights

  - digital libraries!

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What else?

Just storing data?

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SPARQL: 
    SPARQL Protocol and RDF Query Language

(yes, it's recursive)

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DBpedia

<http://dbpedia.org/>
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== Examples

<http://wiki.dbpedia.org/OnlineAccess#h28-5>

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DBtune

<http://dbtune.org/>

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SPARQL examples:

<url:ex01.rq>
<url:ex02.rq>
<url:query.pl>


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The Linking Open Data cloud diagram

<http://richard.cyganiak.de/2007/10/lod/>

Lots of datasets!!!

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Further information

Semantic Web wiki
	<http://semanticweb.org/wiki/Main_Page>
Linked Data
	<http://linkeddata.org/>
SPARQL By Example: A Tutorial
	<http://www.cambridgesemantics.com/2008/09/sparql-by-example/>