Semistructured Data and XML

1
Chapter 29

• Semistructured Data and XML
• Transparencies

2
Chapter - Objectives

• What semistructured data is.
• Concepts of the Object Exchange Model (OEM), a
  model for semistructured data.
• Main language elements of XML.
• Difference between well-formed and valid XML
  documents.
• How Document Type Definitions (DTDs) can be used
  to define the valid syntax of an XML document.

3
Chapter - Objectives

• About other related XML technologies.
• Limitations of DTDs and how the W3C XML Schema
  overcomes these limitations.
• How RDF and RDF Schema provide a foundation for
  processing meta-data.
• Proposals for a W3C Query Language.

4
(No Transcript)
5
Introduction

• In 1998 XML 1.0 was formally ratified by W3C.
• Yet, set to impact every aspect of programming
  including graphical interfaces, embedded systems,
  distributed systems, and database management.
• Already becoming de facto standard for data
  communication within software industry, and is
  quickly replacing EDI systems as primary medium
  for data interchange among businesses.
• Some analysts believe it will become language in
  which most documents are created and stored, both
  on and off Internet.

6
Introduction

• Due to nature of information on Web and inherent
  flexibility of XML, expected that much of the
  data encoded in XML will be semistructured ie.,
  data may be irregular or incomplete, and its
  structure may change rapidly or unpredictably.
• Unfortunately, relational, object-oriented, and
  object-relational DBMSs do not handle data of
  this nature particularly well.

7
Semistructured Data

• Data that may be irregular or incomplete and
  have a structure that may change rapidly or
  unpredictably.
• Semistructured data is data that has some
  structure, but structure may not be rigid,
  regular, or complete.
• Generally, the data does not conform to a fixed
  schema (sometimes terms schema-less or
  self-describing is used to describe such data). .

8
Semistructured Data

• The information normally associated with a schema
  is contained within the data itself.
• In some forms of semistructured data there is no
  separate schema, in others it exists but only
  places loose constraints on the data.
• Unfortunately, relational, object-oriented, and
  object-relational DBMSs do not handle data of
  this nature particularly well.

9
Semistructured Data

• Has gained importance recently for various
  reasons
• may be desirable to treat Web sources like a
  database, but cannot constrain these sources with
  a schema
• may be desirable to have a flexible format for
  data exchange between disparate databases
• emergence of XML as standard for data
  representation and exchange on the Web, and
  similarity between XML documents and
  semistructured data.

10
Example 29.1
11
Example 29.1

• Note, data is not regular
• for John White, hold first and last names, but
  for Ann Beech store single name and also store a
  salary
• for property at 2 Manor Rd, store a monthly rent
  whereas for property at 18 Dale Rd, store an
  annual rent
• for property at 2 Manor Rd, store property type
  (flat) as a string, whereas for property at 18
  Dale Rd, store type (house) as an integer value.

12
Example 29.1
13
Object Exchange Model (OEM)

• Data in OEM is schema-less and self-describing,
  and can be thought of as labeled directed graph
  where nodes are objects, consisting of
• unique object identifier (for example, 7),
• descriptive textual label (street),
• type (string),
• a value (22 Deer Rd).
• Objects are decomposed into atomic and complex
• atomic object contains a value for a base type
  (eg., integer or string) and can be recognized in
  diagram as one that has no outgoing edges.
• All other objects are complex objects whose type
  are a set of object identifiers.

14
Object Exchange Model (OEM)

• A label indicates what the object represents and
  is used to identify the object and to convey the
  meaning of the object, and so should be as
  informative as possible.
• Labels can change dynamically.
• A name is a special label that serves as an alias
  for a single object and acts as an entry point
  into the database (for example, DreamHome is a
  name that denotes object 1).

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Object Exchange Model (OEM)

• An OEM object can be considered as a quadruple
  (label, oid, type, value).
• For example
• Staff, 4, set, 9, 10
• name, 9, string, Ann Beech
• salary, 10, decimal, 12000

16
Semistructured Data - Case StudyObject Exchange
Model
17
OEM Features

• Common model for heterogeneous information
  exchange, self-describing
• Each object

OID
Label
Type
Value

• OID unique identifier or NULL
• Label character string descriptor
• Type atomic data type or set
• Value atomic value or set of object references

• Help pages for labels
• Query language OEM-QL

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Representing Semistructured Data Using OEM
Label
ltcollection, b1, a1, ...gt b1 ltbook, t, agt
t lttitle, Database and ...gt a
ltauthor, n, pgt n ltname, Jeff Ullmangt p
ltpicture, /gifs/ullman.gifgt a1 ltarticle, v,
w, xgt v ltauthor, Gio Wiederholdgt w lttitle,
Mediators in the gt x ltjournal, IEEE
Computergt
Set Value
Memory Addresses
Atomic Value
...
19
An OEM Query Language OEM-QL

• Logic-based language for OEM
• Match object patterns, generate variable
  bindings, construct new OEM objects from existing
  ones
• Get articles published in IEEE Computer
• P -
• Pltarticles ltjournal IEEE Computergtgt
• Get titles of books by Jeff Ullman
• ltanswer_title Tgt -
• ltbook ltauthor Jeff Ullmangt lttitle Tgtgt

20
XML

• Vendors introduced some browser-specific HTML
  tags, making it difficult to develop
  sophisticated, widely viewable Web documents.
• W3C has produced new standard called XML, which
  could preserve general application independence
  that makes HTML portable and powerful.

21
XML

• XML is a restricted version of SGML, designed
  especially for Web documents.
• SGML allows document to be logically separated
  into two one that defines the structure of the
  document (DTD), other containing the text itself.
  
• By giving documents a separately defined
  structure, and by giving authors ability to
  define custom structures, SGML provides extremely
  powerful document management system.
• However, SGML has not been widely adopted due to
  its inherent complexity.

22
XML

• XML attempts to provide a similar function to
  SGML, but is less complex and, at same time,
  network-aware.
• XML retains key SGML advantages of extensibility,
  structure, and validation.
• Since XML is a restricted form of SGML, any fully
  compliant SGML system will be able to read XML
  documents (although the opposite is not true).
• XML is not intended as a replacement for SGML or
  HTML.

23
XML (eXtensible Markup Language)

• origins HTML SGML (ISO Standard, 1986,
  600pp)
• W3C standard (26 pp) XML syntax DTDs
• XML HTML ? presentational tags
• user-defined DTD
  (tagsnesting)
• gt a metalanguage for defining other languages
  via DTDs
• gt XML is more like SGML than HTML
• XML SGML ? complexity, document perspective
  simplicity, data exchange perspective

24
Advantages of XML

• Simplicity
• Open standard and platform/vendor-independent
• Extensibility
• Reuse
• Separation of content and presentation
• Improved load balancing

25
Advantages of XML

• Support for integration of data from multiple
  sources
• Ability to describe data from a wide variety of
  applications
• More advanced search engines
• New opportunities.

26
Why are Database folks so excited about XML?

• XML is just a syntax for (self-describing) data
• This is still exciting because
• No standard syntax for relational data
• With XML, we can
• Translate any legacy data to XML
• Can exchange data in XML format
• Ship over the web, input to any application

27
XML ? machine accessible meaning
This is what a web-page in natural language
looks like for a machine
28
XML ? machine accessible meaning
XML allows meaningful tags to be added toparts
of the text
29
XML ? machine accessible meaning
But to your machine, the tags look like this.
30
XML ? machine accessible meaning
Schemas help.
lt CV gt
by relating common termsbetween documents
private
31
But other people use other schemas
Someone else has one like this.
32
But other people use other schemas
lt CV gt
which dont fit in
private
Moral There is still need for
ontology mapping..
33
An HTML document
34
HTML code

• lttitlegtICS185/ICS180 - Spring, 2003lt/titlegt
• ltbody bgcolor"d0d0ff"gt
• ltH2gtIndexlt/H2gt
• ltULgt
• ltLIgt lta HREF "announcements"gtAnnouncements
  lt/agt
• ltLIgt lta HREF "geninfo"gtCourse Information
  lt/agt
• lt/ULgt
• ltH2gtCourse Informationlt/H2gt
• lta href"geninfo.html"gtGeneral Informationlt/agt.
  The following are a
• few important entries
• ltULgt
• ltligt ltA HREF "geninfo.htmlgoals"gtCourse
  Goalslt/AgtltBRgt
• ltligt ltA HREF "geninfo.htmlcrsenum"gtAbout
  the course numberslt/AgtltBRgt
• lt/ULgt
• lt/bodygt

35
What is the problem?

• To do more fancy things with documents
• need to make their logical structure explicit.
• Otherwise, software applications
• do not know what is what
• do not have any handle over documents.

36
An XML document

• lt?xml version"1.0" ?gt
• ltbibgt
• ltvendor id"id3_4"gt
• ltnamegtQuickBookslt/namegt
• ltemailgtbooksales_at_quickbooks.comlt/emailgt
• ltphonegt1-800-333-9999lt/phonegt
• ltbookgt
• lttitlegtInorganic Chemistrylt/titlegt
• ltpublishergtBrooks/Cole Publishinglt/publish
  ergt
• ltyeargt1991lt/yeargt
• ltauthorgt
•   ltfirstnamegtJameslt/firstnamegt
•   ltlastnamegtBowserlt/lastnamegt
• lt/authorgt
• ltpricegt43.72lt/pricegt
• lt/bookgt
• lt/vendorgt
• lt/bibgt

37
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
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
HTML describes presentation
XML describes content
38
What is XML?

• eXtensible Markup Language
• Data are identified using tags (identifiers
  enclosed in angle brackets lt...gt)
• Collectively, the tags are known as markup
• XML tags tell you what the data means, rather
  than how to display it

39
XML versus relational

• Relational structured
• XML semi-structured
• Plain text file unstructured

40
How does XML work?

• XML allows developers to write their own Document
  Type Definitions (DTD)
• DTD is a markup languages rule book that
  describes the sets of tags and attributes that is
  used to describe specific content
• If you want to use a certain tag, then it must
  first be defined in DTD

41
Key Components in XML

• Three generic components, and one customizable
  component

XML Content
XML Parser
Application
DTD Rules
42
Meta Markup Language

• Not a language but a way of specifying other
  languages
• Meta-markup language gives the rules by which
  other markup languages can be written
• Portable - platform independent

43
Markup Languages

• Presentation based
• Markup languages that describe information for
  presentation for human consumption
• Content based
• Describe information that is of interest to
  another computer application

44
HTML and XML

• HTML tag says "display this data in bold font"
• ltbgt...lt/bgt
• XML tag acts like a field name in your program
• It puts a label on a piece of data that
  identifies it
• ltmessagegt...lt/messagegt

45
HTML vs. 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

• 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

Self-describing -Schema info part of the
data -Good for data exchange (albeit
baroque for storage)
46
Simple Example

• XML data for a messaging application
• ltmessagegt
• lttogtyou_at_yourAddress.comlt/togt ltfromgtme_at_myAddress.c
  omlt/fromgt lttextgt Why is it good? Let me count
  the ways... lt/textgt
• lt/messagegt

47
Element

• Data between the tag and its matching end tag
  defines an element of the data
• Comment
• lt!-- This is a comment --gt

48
Example

• lt!-- Using attributes   --gt
• ltmessage to"you_at_yourAddress.com"
  from"me_at_myAddress.com"gt 
• lttextgtWhty is it good? Let me count the
  ways...lt/textgt  
• lt/messagegt

49
Attributes

• Tags can also contain attributes
• Attributes contain additional information
  included as part of the tag, within the tag's
  angle brackets
• Attribute name is followed by an equality sign
  and the attribute value

50
Other Basics

• White space is essentially irrelevant
• Commas between attributes are not ignored - if
  present, they generate an error
• Case sensitive message and MESSAGE are
  different

51
Well Formed XML

• Every tag has a closing tag
• XML represents hierarchical data structures
  having one tag to contain others
• Tags have to be completely nested
• Correct
• ltmessagegt..lttogt..lt/togt..lt/messagegt
• Incorrect
• ltmessagegt..lttogt..lt/messagegt..lt/togt

52
Empty Tag

• Empty tag is used when it makes sense to have a
  tag that stands by itself and doesn't enclose any
  content - a "flag"
• You can create an empty tag by ending it with /gt
• ltflag/gt

53
Example

• ltmessage to"you_at_yourAddress.com"
  from"me_at_myAddress.com" subjectXML is good"gt
  ltflag/gt lttextgt Whty is it good? Let me count the
  ways...
• lt/textgt
• lt/messagegt

54
Tree representation

• ltBOOKSgt
• ltbook id123 loclibrarygt
• ltauthorgtHulllt/authorgt
• lttitlegtCalifornialt/titlegt
• ltyeargt 1995 lt/yeargt
• lt/bookgt
• ltarticle id555 ref123gt
• ltauthorgtSult/authorgt
• lttitlegt Purduelt/titlegt
• lt/articlegt
• lt/BOOKSgt

Hull
55
Prolog in XML Files

• XML file always starts with a prolog
• The minimal prolog contains a declaration that
  identifies the document as an XML document
• lt?xml version"1.0"?gt
• The declaration may also contain additional
  information
• version - version of the XML used in the data
• encoding - Identifies the character set used
• standalone - whether the document references an
  external entity or data type specification

56
Detailed Example of XML File

• simple version of the kind of XML data you could
  use for a slide presentation
• You can use your text editor to create the data
• Step 1 create a file named slideSample01.xml
• Step 2 write the declaration, which identifies
  the file as an XML document
• lt?xml version'1.0' encoding'us-ascii'?gt

57
Defining the Root Element

• Step 3 Adding a comment
• lt!-- A SAMPLE set of slides --gt
• Step 4 Defining the Root Element
• ltslideshowgt
• lt/slideshowgt
• After the declaration, every XML file defines
  exactly one element, known as the root element
• Any other elements in the file are contained
  within that element

58
Attributes

• A slide presentation has a title
• ...
• ltslideshow
• title"Sample Slide Show"gt
• lt/slideshowgt

59
Adding Nested Elements

• Step 5 Adding Nested Elements
• ltslideshow
• ...
• lt!-- TITLE SLIDE --gt
• ltslide title"Title of Talk"/gt
• lt!-- TITLE SLIDE --gt
• ltslide type"all"gt
• lttitlegtIntroduction to XML lt/titlegt
• lt/slidegt
• lt/slideshowgt

60
Attribute vs. Element

• type of the slide is defined as an attribute
• Slides could be earmarked for a mostly technical
  or mostly executive audience with type"tech" or
  type"exec", or identified as suitable for both
  with type"all
• title element is defined as an element
• The title is something the audience will see
• So it is an element
• The type is something that never gets presented
• So it is an attribute

61
Adding Text

• Step 6 Adding Text
• ltslideshowgt
• lt!-- OVERVIEW --gt
• ltslide type"all"gt lttitlegtOverviewlt/titlegt
• ltitemgtWhy is XML great?lt/itemgt
• ltitemgtWho uses it?lt/itemgt
• lt/slidegt
• lt/slideshowgt

62
Adding an Empty Element

• Step 7 Adding an Empty Element
• ltslideshowgt
• lt!-- OVERVIEW --gt
• ltslidegt
• lt!-- define an empty list item --gt
• ltitem/gt
• lt/slidegt
• lt/slideshowgt

63
Complete Example

•  lt?xml version"1.0" encoding"us-ascii" ?gt
• lt!-- A SAMPLE set of slides   --gt
• ltslideshow title"Sample Slide Show"gt
• lt!-- TITLE SLIDE   --gt
• ltslide type"all"gt 
• lttitlegtIntroduction to CMLlt/titlegt
•   lt/slidegt
• lt!-- OVERVIEW   --gt
• ltslide type"all"gt
•   lttitlegtOverviewlt/titlegt
•  ltitemgtWhy is XML great?lt/itemgt
•   ltitem /gt  
• lt/slidegt
•  lt/slideshowgt

64
XML Parsing IE Example
65
Processing Instructions

• An XML file can also contain processing
  instructions that give commands or information to
  an application that is processing the XML data
• lt?target instructions?gt
• target is the name of the application that is
  expected to do the processing
• instructions is a string of characters that
  embodies the information or commands for the
  application to process

66
XML
67
XML -Elements

• Elements, or tags, are most common form of
  markup.
• First element must be a root element, which can
  contain other (sub)elements.
• XML document must have one root element
  (ltSTAFFLISTgt. Element begins with start-tag
  (ltSTAFFgt) and ends with end-tag (lt/STAFFgt).
• XML elements are case sensitive
• An element can be empty, in which case it can be
  abbreviated to ltEMPTYELEMENT/gt.
• Elements must be properly nested.

68
XML - Attributes

• Attributes are name-value pairs that contain
  descriptive information about an element.
• Attribute is placed inside start-tag after
  corresponding element name with the attribute
  value enclosed in quotes.
• ltSTAFF branchNo B005gt
• Could also have represented branch as subelement
  of STAFF.
• A given attribute may only occur once within a
  tag, while subelements with same tag may be
  repeated.

69
Data Type Definition (DTD)

• DTD specifies the types of tags that can be
  included in the XML document
• it defines which tags are valid, and in what
  arrangements
• where text is expected, letting the parser
  determine whether the whitespace it sees is
  significant or ignorable
• An optional part of the document prolog

70
XML document and DTD
XML DTD
Slideshow

Slideshow
Slide

slide
slide
item
title
title
item
DB
item
title
item
item1
item2

• lt?xml version'1.0' encoding'us-ascii'?gt
• lt!-- DTD for a simple "slide show".--gt
• lt!ELEMENT slideshow (slide)gt
• lt!ELEMENT slide (title, item)gt
• lt!ELEMENT title (PCDATA)gt
• lt!ELEMENT item (PCDATA item) gt

AI
item3
XML Document
71
Detailed DTD Example

• Step 1 Create a file named slideshow.dtd
• Step 2 Enter an XML declaration
• lt?xml version'1.0' encoding'us-ascii'?gt
• lt!-- DTD for a simple "slide show". --gt
• Step 3 Specify contains of a slideshow element
• lt!- slideshow contains 1 slide elements --gt
• lt!ELEMENT slideshow (slide)gt

72
Qualifiers

• lt?xml version'1.0' encoding'us-ascii'?gt
• lt!-- DTD for a simple example. --gt
• lt!ELEMENT slideshow (slide)gt
• slideshow element contains slide elements and
  nothing else

73
Grouping multiple items

• ((image, title))
• Every image element must be paired with a title
  element
• Plus sign applies to the image/title pair to
  indicate that one or more pairs of the specified
  items can occur

74
Defining Text and Nested Elements

• Step 4 Defining Text and Nested Elements
• lt!ELEMENT slide (title, item)gt
• lt!ELEMENT title (PCDATA)gt
• lt!ELEMENT item (PCDATA item) gt
• Text Parsed Character DATA (PCDATA)
• "" that precedes PCDATA indicates that what
  follows is a special word, rather than an element
  name

75
Complete Example

• lt?xml version'1.0' encoding'us-ascii'?gt
• lt!-- DTD for a simple "slide show".--gt
• lt!ELEMENT slideshow (slide)gt
• lt!ELEMENT slide (title, item)gt
• lt!ELEMENT title (PCDATA)gt
• lt!ELEMENT item (PCDATA item) gt

76
Attribute Types

• (PCDATA item)
• Vertical bar () indicates an or condition
• In this case, either PCDATA or an item can occur

77
What you cannot do?

• Double-definition for an item element doesn't
  work
• lt!ELEMENT item (PCDATA) gt
• lt!ELEMENT item (PCDATA, item) gt
• Produces a "duplicate definition" warning
• The second definition is ignored

78
XML Names and NMTOKEN

• Name Characters are letters, digits, hyphens,
  underscores, colons or full stops.
• An NMTOKEN is any collection of Name Characters
• NMTOKENS is any list of NMTOKENs separated by
  white space (space, tab, newline etc.)
• Case is significant PERSON and person are
  distinct names
• Attribute and Element names must be (a subset of)
  NMTOKEN with restriction
• Names cannot begin with a digit
• Names cannot begin with xml (or any variant
  gotten by case changes) system will use this
  prefix

79
Element Declarations EMPTY

• Keyword ELEMENT Introduces a new
  elementlt!ELEMENT NAME CONTENT_MODELgt
• Element name must begin with a letter, and may
  additionally contain digits and some
  punctuations, i.e. ., -, _, and as we
  described earlier under NMTOKEN
• If an element can hold no child elements, and
  also no text, then it is known as empty element
  and denoted by EMPTY for CONTENT_MODEL
• This seems trivial but it isnt because the
  present or absence of this element in an XML file
  can be used as a flag
• As an example we can find several in HTML such as
  HR and IMG which never have children and include
  no text. Here we would writelt!ELEMENT HR EMPTYgt
  and then ltHR/gt or ltHRgtlt/HRgt generates a
  horizontal line
• EMPTY ELEMENTS can have attributes such as the
  SRC attribute in ltIMG/gt to specify source of
  image.

80
Element Declarations ANY

• An element declared to have a content of ANY may
  contain all of the other elements declared in the
  DTD
• This is not quite the same as no DTD for the file
• lt!DOCTYPE fred lt!ELEMENT fred ANY gtgt
• ltfredgt ltpeoplegtMe and Yoult/peoplegt ltpeoplegtThem
  lt/peoplegtlt/fredgt
• Gets an error due to presence of ltpeoplegt tag
• Adding lt!ELEMENT people ANY gt inside DTD
  declaration produces a valid document.

81
Entities

• The DTD of an XML document can contain entity
  declarations. These are like macro substitutions
  in other languages.
• ENTITYs are defined in DTD and consist of
  several flavors
• General Entities are referenced as EntName
• Parameter Entities are referenced as Entname
• We have already seen the character entities
• amp for
• apos for
• gt for gt
• lt for lt
• quot for
• These are built in but you could add other such
  entities with
• lt!ENTITY aitself A gt and aitself would be
  replaced by A

82
General Entities

• As another example, we can use in DTDlt!ENTITY
  TODAY May 12 2003 gt andltcommentgtTODAY was
  very quiet in Irvinelt/commentgtis parsed as
  ltcommentgtMay 12 2003 was very quiet in
  Irvinelt/commentgt
• General Entity references can be nested inside a
  DTD, e.g., one can write lt!ENTITY YEAR 2003 gt
  lt!ENTITY TODAY May 12 YEAR gt
• However one must use Parameter Entities and not
  General Entities for macro substitution in other
  DTD declarations like lt!ATTLIST and lt!ELEMENT
• Parameter entities are defined as inlt!ENTITY
  CUSTARDTAGS (NAME,DATE,ORDERS) gt

83
Parameter Entities

• lt!ENTITY peopletags (firstname,lastname,dateofbi
  rth) gtlt!ELEMENT student peopletags gt
  lt!ELEMENT teacher peopletags gt lt!ELEMENT
  administrator peopletags gt
• Defines a bunch of people ELEMENTS to have the
  same child elements
• Parameter entities are even more commonly used
  for attributes because almost always several
  ELEMENTS share the same attributes (with often a
  basic set being augmented in different ways for
  different ELEMENTS)
• This basic set can be set in a parameter Entity

84
Defining Implied Attributes

• Attributes must be declared in the DTD to be able
  to be used
• Implied means that this attribute optional and
  there is no default value
• lt!ELEMENT population (PCDATA)gt
• lt!ATTLIST population year CDATA IMPLIEDgt
• The attribute year can be defined or undefined in
  the element population. Valid Examples
• ltpopulation year2000gt80lt/populationgt
• ltpopulationgt80lt/populationgt

85
Defining Required Attributes

• lt!ELEMENT population (PCDATA)gt lt!ATTLIST
  population year REQUIREDgt
• The population must contain a year attribute
• ltpopulation year1996gt80lt/populationgt
• lt!ELEMENT population (PCDATA)gt lt!ATTLIST
  population year (20002001) REQUIREDgt
• The population must contain a year attribute of
  2000 or 2001
• ltpopulation year2000gt80lt/populationgt
• No quotes on the enumeration values

86
Defining Default Attributes

• lt!ELEMENT population (PCDATA)gt lt!ATTLIST
  population year CDATA 2000gt
• All these are valid
• ltpopulation year2001gt80lt/populationgt
• ltpopulation year2000gt80lt/populationgt
• ltpopulationgt80lt/populationgt

87
Defining Fixed Attributes

• lt!ELEMENT population (PCDATA)gt lt!ATTLIST
  population year CDATA FIXED 2000gt
• Invalid ltpopulation year2001gt80lt/populationgt
• Valid ltpopulation year2000gt80lt/populationgt
• Valid ltpopulationgt80lt/populationgt

88
Defining Unique Attributes

• lt!ELEMENT animal (name)gt
• lt!ATTLIST animal code ID REQUIREDgt
• The code attribute has to be unique in the XML
  document
• ltanimal codeT50gtltnamegtLionlt/namegt lt/animalgt
  
  ltanimal codeT51gtltnamegtRabbitlt/namegt lt/animalgt

89
Referring Unique Attributes

• lt!ELEMENT website (url)gt
  lt!ATTLIST website animal_refer IDREF REQUIREDgt
• animal_refer attribute refers to previous ID
  attribute defined
• ltwebsite animal_referT50gt
  lturlgthttp//www.lions.comlt/urlgt
  lt/websitegt

90
Referring Multiple Unique Attributes

• lt!ELEMENT website (url)gt
  lt!ATTLIST website contents IDREFS REQUIREDgt
• contents attribute contain series of IDs
• ltwebsite contentsT50 T51gt
  lturlgthttp//www.animals.comlt/urlgt
  lt/websitegt

91
XML Example - the DTD

• lt!ELEMENT addressBook (person)gt
• lt!ELEMENT person (name, email, link?) gt
• lt!ATTLIST person id ID REQUIRED gt
• lt!ATTLIST person gender (malefemale) IMPLIEDgt
• lt!ELEMENT name (PCDATA(family,given))gt
• lt!ELEMENT family (PCDATA)gt
• lt!ELEMENT given (PCDATA)gt
• lt!ELEMENT email (PCDATA)gt
• lt!ELEMENT link EMPTY gtlt!ATTLIST link manager
  IDREF IMPLIED
  subordinates IDREF IMPLIEDgt

92
DOCTYPE declarations

• Internal local definition of DTD
• External to an external file
• Can combine both

93
Internal DTD

• lt?xml version"1.0" standalone"yes" ?gt
• lt!--open the DOCTYPE declaration -
• the open square bracket indicates an internal
  DTD--gt
• lt!DOCTYPE foo
• lt!--define the internal DTD--gt
• lt!ELEMENT foo (PCDATA)gt
• lt!--close the DOCTYPE declaration--gt
• gt
• ltfoogtHello World.lt/foogt

94
Internal DTD rules

• The document type declaration must be placed
  between the XML declaration and the first element
  (root element) in the document .
• The keyword DOCTYPE must be followed by the name
  of the root element in the XML document .
• The keyword DOCTYPE must be in upper case .

95
External DTD

• Useful for creating a common DTD that can be
  shared between multiple documents.
• Any changes that are made to the external DTD
  automatically updates all the documents that
  reference it.
• Two types private, and public.
• Rules
• If any elements, attributes, or entities are used
  in the XML document that are referenced or
  defined in an external DTD, standalone"no" must
  be included in the XML declaration .

96
"Private" External DTDs

• Identified by the keyword SYSTEM
• Intended for use by a single author or group of
  authors.
• Example
• lt!DOCTYPE root_element SYSTEM "DTD_location"gt
• where DTD_location is relative or absolute URL
  (such as
• http/ and file/).

97
"Private" External DTDs (cont)

• XML document
• lt?xml version"1.0" standalone"no" ?gt
• lt!DOCTYPE document SYSTEM "subjects.dtd"gt
• ltdocumentgt lt/documentgt
• subjects.dtd
• lt!ELEMENT document gt

98
Public" External DTDs

• Identified by the keyword PUBLIC
• Intended for broad use.
• lt!DOCTYPE root_element PUBLIC "DTD_name"
  "DTD_location"gt where
• DTD_location relative or absolute URL
• DTD_name follows the syntax
• "prefix//owner_of_the_DTD// description_of_the_D
  TD//ISO 639_language_identifier
• "DTD_location" is used to find the public DTD if
  it cannot be located by the "DTD_name".

99
Public" External DTDs (cont)

• lt?xml version"1.0" standalone"no" ?gt
• lt!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0
  Transitional//EN" "http//www.w3.org/TR/REC-html40
  /loose.dtd"gt
• ltHTMLgt
• ltHEADgt
• ltTITLEgtA typical HTML filelt/TITLEgt
• lt/HEADgt
• ltBODYgt
• lt/BODYgt
• lt/HTMLgt

100
Public" External DTDs (cont)

• Valid DTD_name Prefix
• ISO The DTD is an ISO standard. All ISO
  standards are approved.
• The DTD is an approved non-ISO standard.
• - The DTD is an unapproved non-ISO standard.

101
Combining Internal and External DTDs

• A document can use both internal and external DTD
  subsets.
• The internal DTD subset is specified between the
  square brackets of the DOCTYPE declaration.
• The declaration for the external DTD subset is
  placed before the square brackets immediately
  after the SYSTEM keyword.
• Declaring an ELEMENT with the same name in both
  the internal and external DTD subsets is invalid

102
Example

• lt?xml version"1.0" standalone"no" ?gt
• lt!DOCTYPE document SYSTEM "subjects.dtd"
• lt!ATTLIST assessment assessment_type (exam
  assignment prac)gt
• lt!ELEMENT results (PCDATA)gt
• gt
• subjects.dtd
• lt!ELEMENT document (title,subjectID,subjectname,p
  rerequisite?, classes,assessment,syllabus,textbook
  s)gt
• lt!ELEMENT prerequisite (subjectID,subjectname)gt

103
DTD Validation

• A XML content can be well-formed but invalid
  under DTD rules
• e.g. DTD rule lt!ELEMENT name (PCDATA)gt
• Acceptable ltnamegt Giancarlo Succi lt/namegt
• Unacceptable
• ltnamegt
• ltfirst_namegt Giancarlo lt/first_namegt
• ltlast_namegt Succi lt/last_namegt
• lt/namegt

104
Beyond DTDs

• DTD limitations
• Simple document structures
• Lack of real datatypes
• Advanced schema languages
• XML Schema
• Relax NG

105
References

• http//www.java.sun.com/xml/docs/tutorial/TOC.html
  
• http//www.xml.com/pub/a/1999/09/expat/index.html
• http//xmlfiles.com/dtd/dtd_attributes.asp
• http//xmlwriter.net/xml_guide/doctype_declaration
  .shtml