Semistructured Data and XML and RDF

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Semistructured Data and XML and RDF

• Overview

Some of the materials of this lecture are from
Prof. Dan Suciu, Univ. of Washington and John
Punin, Rensselaer Polytechnic Inst.
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Overview

• Semistructured Data
• Model
• Syntax
• Comparison with relational data
• XML
• Motivation
• Syntax, DTDs
• RDF
• Motivation
• Syntax, RDFS
• Semantics

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Semistructured Data

• Schemaless, self-describing
• Labeled tree

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XML v.s. Semistructured Data

• both described best by a graph
• both are schema-less, self-describing

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More Differences

• XML is ordered, ssd is not
• XML can mix text and elements
• lttalkgt Making Java easier to type and easier
  to type
• ltspeakergt Phil Wadler lt/speakergt
• lt/talkgt
• XML has lots of other stuff entities, processing
  instructions, etc.

These differences make XML data management harder
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XML

• a W3C standard to complement HTML
• origins structured text SGML
• motivation
• HTML describes presentation
• XML describes content
• http//www.w3.org/TR/2000/REC-xml-20001006
  (version 2, 10/2000)

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From HTML to XML
HTML describes the presentation
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HTML

• lth1gt Bibliography lt/h1gt
• ltpgt ltigt Foundations of Databases lt/igt
• Abiteboul, Hull, Vianu
• ltbrgt Addison Wesley, 1995
• ltpgt ltigt Data on the Web lt/igt
• Abiteoul, Buneman, Suciu
• ltbrgt Morgan Kaufmann, 1999

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XML

• ltbibliographygt
• ltbookgt lttitlegt Foundations lt/titlegt
• ltauthorgt Abiteboul lt/authorgt
• ltauthorgt Hull lt/authorgt
• ltauthorgt Vianu lt/authorgt
• ltpublishergt Addison Wesley
  lt/publishergt
• ltyeargt 1995 lt/yeargt
• lt/bookgt
• lt/bibliographygt

XML describes the content
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XML Terminology

• tags book, title, author,
• start tag ltbookgt, end tag lt/bookgt
• elements ltbookgtltbookgt,ltauthorgtlt/authorgt
• elements are nested
• an XML document single root element

well formed XML document if it has matching tags
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More XML Attributes

• ltbook price 55 currency USDgt
• lttitlegt Foundations of Databases lt/titlegt
• ltauthorgt Abiteboul lt/authorgt
• ltyeargt 1995 lt/yeargt
• lt/bookgt

attributes are alternative ways to represent data
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More XML Oids and References

• ltperson ido555gt ltnamegt Jane lt/namegt lt/persongt
• ltperson ido456gt ltnamegt Mary lt/namegt
• ltchildren
  idrefo123 o555/gt
• lt/persongt
• ltperson ido123 mothero456gtltnamegtJohnlt/namegt
• lt/persongt

oids and references in XML are just syntax
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XML Namespaces

• http//www.w3.org/TR/REC-xml-names (1/99)
• name prefixlocalpart

ltbook xmlnsisbnwww.isbn-org.org/defgt
lttitlegt lt/titlegt ltnumbergt 15 lt/numbergt
ltisbnnumbergt . lt/isbnnumbergt lt/bookgt
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XML Namespaces

• syntactic ltnumbergt , ltisbnnumbergt
• semantic provide URL for schema

lttag xmlnsmystyle http//gt
ltmystyletitlegt
lt/mystyletitlegt ltmystylenumbergt
lt/taggt
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XLink

• Generalizes HTMLs href
• Many types simple, extended, locator, ...
• Discuss only simple links

ltperson xmlnsxlinkhttp///.w3.org/1999/xlink
xlinktypesimple
xlinkhrefhttp//a.b.c/myhomepage.html
xlinktitleThe Homepage
xlinkshowreplace
xlinkactuateonRequestgt ..... lt/persongt
required attributes
optional attributes
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XPointer

• An extension of XPath
• Usage
• hrefwww.a.b.c/document.xmlxpointerExpr
• An xpointer expression points to
• A point
• A range

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XMLDocument Type Definitions

• part of the original XML specification
• an XML document may have a DTD
• terminology for XML
• well-formed if tags are correctly closed
• valid if it has a DTD and conforms to it
• validation is useful in data exchange

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XMLDocument Type Definitions

• part of the original XML specification
• an XML document may have a DTD
• terminology for XML
• well-formed if tags are correctly closed
• valid if it has a DTD and conforms to it
• validation is useful in data exchange

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Very Simple DTD
Example of valid XML document
ltcompanygt ltpersongt ltssngt 123456789 lt/ssngt
ltnamegt John lt/namegt
ltofficegt B432 lt/officegt
ltphonegt 1234 lt/phonegt lt/persongt
ltpersongt ltssngt 987654321 lt/ssngt
ltnamegt Jim lt/namegt
ltofficegt B123 lt/officegt lt/persongt
ltproductgt ... lt/productgt ... lt/companygt
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Content Model

• Element content what we can put in an element
  (aka content model)
• Content model
• Complex a regular expression over other
  elements
• Text-only PCDATA
• Empty EMPTY
• Any ANY
• Mixed content (PCDATA A B C)
• (i.e. very restrictied)

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Attributes in DTDs
lt!ELEMENT person (ssn, name, office,
phone?)gt lt!ATTLIS person age CDATA
REQUIRED id
ID REQUIRED
manager IDREF REQUIRED
manages IDREFS
REQUIRED gt
ltperson age25 idp29432
managerp48293 managesp34982
p423234gt ltnamegt ....lt/namegt
... lt/persongt
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Attributes in DTDs

• Kind
• REQUIRED
• IMPLIED optional
• value default value
• value FIXED the only value allowed

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XML Schemas

• http//www.w3.org/TR/xmlschema-1/10/2000
• generalizes DTDs
• uses XML syntax
• two documents structure and datatypes
• http//www.w3.org/TR/xmlschema-1
• http//www.w3.org/TR/xmlschema-2
• XML-Schema is very complex
• often criticized
• some alternative proposals

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RDF

• RDF Model and Syntax
• Language to describe resources
• Use metadata (data about data) to describe Web
  resources
• Provides interoperability between applications
  that exchange machine-understandable information
  on the Web
• Use XML as a syntax

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RDF Triple

• Resources - Things being described by RDF
  expressions. Resources are always named by URIs
• HTML Document
• Specific XML element within the document source.
• Collection of pages
• Properties - Specific aspect, characteristic,
  attribute or relation used to describe a resource
  
• Creator
• Title
• Name
• Property Value the corresponding value of the
  property related to the resource
• John Smith
• Science Fiction

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RDF Schema

• Basic vocabulary to describe RDF
• Defines properties of the resources (e.g., title,
  author, subject, etc)
• Defines kinds of resources being describes
  (books, Web pages, people, etc)
• XML Schema gives specific constraints on the
  structure of an XML document
• RDF Schema provides information about the
  interpretation of the RDF statements

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RDF Entailment

• Generate new RDF facts
• Schema and instance information
• Entailment Rules