Module 3 XML Schema

1
Module 3XML Schema
2
Recapitulation (Module 2)

• XML as inheriting from the Web history
• SGML, HTML, XHTML, XML
• XML key concepts
• Documents, elements, attributes, text
• Order, nested structure, textual information
• Namespaces
• XML usage scenarios
• Financial, medical, Web Services, blogs, etc
• communication data, meta data, documents
• DTDs and the need for describing the structure
  of an XML file
• Next XML Schemas

3
Limitations of DTDs

• DTDs describe only the grammar of the XML file,
  not the detailed structure and/or types
• This grammatical description has some obvious
  shortcomings
• we cannot express that a length element must
  contain a non-negative number (constraints on the
  type of the value of an element or attribute)
• The unit element should only be allowed when
  amount is present (co-occurrence constraints)
• the comment element should be allowed to appear
  anywhere (schema flexibility)
• There is no subtyping / inheritance (reuse of
  definitions)
• There are no composite keys (referential
  integrity)

4
Overview XML Schema

• ComplexTypes and SimpleTypes
• ComplexType correspond to Records
• string is an example of a SimpleType
• Built-in and user-defined Types
• ComplexTypes are always user-defined
• Elements have complexTypes or simpleTypes
  Attributes have simpleTypes
• Type of Root element of a document is global
• (almost) downward comptable with DTDs
• Schemas are XML Documents (Syntax)
• Namespaces etc. are part of XML Schemas

5
Example Schema

• lt?xml version1.0 ?gt
• ltxsdschema xmlnsxsdhttp//w3.org/2001/XMLSchem
  agt
• ltxsdelement namebook typeBookType/gtltxsd
  complexType nameBookTypegt
• ltxsdsequencegt
• ltxsdelement nametitle
  typexsdstring/gt
• ltxsdelement nameauthor typePersonType m
  inOccurs1 maxOccursunbounded/gt
• ltxsdcomplexType namePersonTypegt
  ltxsdsequencegt ... ltxsdsequencegt
• lt/xsdcomplexTypegt
• ltxsdelement namepublisher
  typexsdanyType/gt
• lt/xsdsequencegt
• lt/xsdcomplexTypegt
• lt/xsdschemagt

6
Example Schema

• lt?xml version1.0 ?gt
• ltxsdschema xmlnsxsdhttp//w3.org/2001/XMLSchem
  agt
• ...
• lt/xsdschemagt
• Schema in a separate XML Document
• Vocabulary of Schema defined in special
  Namespace. Prefix xsd is commonly used
• There is a Schema for Schemas (dont worry!)
• schema Element is always the Root

7
Example Schema

• ltxsdelement namebook typeBookType/gt
• element Element in order to declare elements
• name defines the name of the element.
• type defines the type of the element
• Declarations under schema are global
• Global element declarations are potential roots
• Example book is the only global element, root
  element of a valid document must be a book.
• The type of a book is BookType (defined next).

8
Example Schema

• ltxsdcomplexType nameBookTypegt
• ltxsdsequencegt
• ...
• lt/xsdsequencegt
• lt/xsdcomplexTypegt
• User-defined complex type
• Defines a sequence of sub-elements
• Attribute name specifies name of Type
• This Typedefinition is global.Type can be used
  in any other definition.

9
Example Schema

• ltxsdsequencegt
• ltxsdelement nametitle
  typexsdstring/gt
• lt/xsdsequencegt
• Local element declaration within a complex type
• (title cannot be root element of documents)
• name and type as before
• xsdstring is built-in type of XML Schema

10
Example Schema

• ltxsdelement nameauthor typePersonType m
  inOccurs1 maxOccursunbounded/gt
• Local element declaration
• PersonType is user-defined type
• minOccurs, maxOccurs specify cardinality of
  author Elements in BookType.
• Default minOccurs1, maxOccurs1

11
Example Schema

• ltxsdcomplexType namePersonTypegt ltxsdsequenc
  egt
• ltxsdelement namefirst typexsdstring/gt
• ltxsdelement namelast typexsdstring/gt
• ltxsdsequencegtlt/xsdcomplexTypegt
• Local type definition
• PersonType may only be used inside the scope of
  the definition of BookType.
• The same syntax as for BookType.

12
Example Schema

• ltxsdelement namepublisher
  typexsdanyType/gt
• Local element declaratation
• Every book has exactly one publisher
  minOccurs, maxOccurs by default 1
• anyType is built-in Type
• anyType allows any content
• anyType is default type. Equivalent
  definition
• ltxsdelement namepublisher /gt

13
Example Schema

• lt?xml version1.0 ?gt
• ltxsdschema xmlnsxsdhttp//w3.org/2001/XMLSchem
  agt
• ltxsdelement namebook typeBookType/gtltxsd
  complexType nameBookTypegt
• ltxsdsequencegt
• ltxsdelement nametitle
  typexsdstring/gt
• ltxsdelement nameauthor typePersonType m
  inOccurs1 maxOccursunbounded/gt
• ltxsdcomplexType namePersonTypegt
• ltxsdsequencegt ... ltxsdsequencegt
• lt/xsdcomplexTypegt
• ltxsdelement namepublisher
  typexsdanyType/gt
• lt/xsdsequencegt
• lt/xsdcomplexTypegt
• lt/xsdschemagt

14
Valid Document

• lt?xml version1.0gt
• ltbookgt
• lttitlegtDie Wilde Wutzlt/titlegt
• ltauthorgtltfirstgtD.lt/firstgt
• ltlastgtK.lt/lastgtlt/authorgt
• ltpublishergt Addison Wesley,
• ltstategtCAlt/stategt, USA
• lt/publishergt
• lt/bookgt

15
Valid Document
Root is book

• lt?xml version1.0gt
• ltbookgt
• lttitlegtDie Wilde Wutzlt/titlegt
• ltauthorgtltfirstgtD.lt/firstgt
• ltlastgtK.lt/lastgtlt/authorgt
• ltpublishergt Addison Wesley,
• ltstategtCAlt/stategt, USA
• lt/publishergt
• lt/bookgt

16
Valid Document

• lt?xml version1.0gt
• ltbookgt
• lttitlegtDie Wilde Wutzlt/titlegt
• ltauthorgtltfirstgtD.lt/firstgt
• ltlastgtK.lt/lastgtlt/authorgt
• ltpublishergt Addison Wesley,
• ltstategtCAlt/stategt, USA
• lt/publishergt
• lt/bookgt

Exactly one title of Type string
17
Valid Document

• lt?xml version1.0gt
• ltbookgt
• lttitlegtDie Wilde Wutzlt/titlegt
• ltauthorgtltfirstgtD.lt/firstgt
• ltlastgtK.lt/lastgtlt/authorgt
• ltpublishergt Addison Wesley,
• ltstategtCAlt/stategt, USA
• lt/publishergt
• lt/bookgt

Subelements In right order
At least one author of Type PersonType
One publisherwith arbitrary content.
18
Schema Validation

• Conformance Test
• Result true or false
• Infoset Contribution
• Annotate Types
• Set Default Values
• Result new instance of the data model
• Tools Xerces (Apache)
• Theory Graph Simulation Algorithms
• Validation is a-posteri explicit - not implicit!

19
Global vs. Local Declarations

• Instances of global element declarations are
  potential root elements of documents
• Global declarations can be referencedltxsdschema
  xmlnsxsd...gt ltxsdelement namebook
  typeBookType/gt ltxsdelement namecomment
  typexsdstring/gt ltxsdComplexType
  nameBookTypegt ... ltxsdelement
  refcomment minOccurs0/gt...
• Constraints
• ref not allowed in global declarations
• No minOccurs, maxOccurs in global Decl.

20
Attribute Declarations

• Attributes may only have a SimpleType
• SimpleTypes are, e.g., string (more later)
• Attribute declarations can be global
• Reuse declarations with ref
• Compatible to Attribute lists in DTDs
• Default values possible
• Required and optional attributes
• Fixed attributes
• (In addition, there are prohibited attributes)

21
Attribute Declarations

• ltxsdcomplexType nameBookTypegt
• ltxsdsequencegt ... lt/xsdsequencegt
• ltxsdattribute nameisbn typexsdstring
  
• userequired /gt
• ltxsdattribute nameprice typexsddecimal
• useoptional /gt
• ltxsdattribute namecurr typexsdstring
• fixedEUR /gt
• ltxsdattribute nameindex typexsdidrefs
• default /gt
• lt/xsdcomplexTypegt

22
Anonymous Types

• PersonType need not be named
• ltxsdcomplexType nameBookTypegt
• ...
• ltxsdelement nameauthorgt
• ltxsdcomplexTypegt
• ltxsdsequencegt
• ltxsdelement namefirst
  typexsdstring/gt
• ltxsdelement namelast
  typexsdstring/gt
• lt/xsdsequencegt lt/xsdcomplexTypegt
• lt/xsdelementgt ...

23
Simple Elements Attributes

• ltxsdelement namepricegt
• ltxsdcomplexTypegt
• ltxsdsimpleContentgt
• ltxsdextension base xsddecimal gt
• ltxsdattribute namecurr
  typexsdstring/gt
• lt/xsdextensiongt
• lt/xsdsimpleContentgt
• lt/xsdcomplexTypegt lt/xsdelementgt
• Valid Instance ltprice currUSD gt69.95lt/pricegt

24
Element Attributes, no Content

• ltxsdelement namepricegt
• ltxsdcomplexTypegt
• ltxsdattribute namecurr
  typexsdstring/gt
• ltxsdattribute nameval typexsddecimal/
  gt
• lt/xsdcomplexTypegt
• lt/xsdelementgt
• Valid Instance
• ltprice currUSD val69.95 /gt

25
Pre-defined SimpleTypes

• Numeric Values
• Integer, Short, Decimal, Float, Double,
  HexBinary, ...
• Date, Timestamps, Periods
• Duration, DateTime, Time, Date, gMonth, ...
• Strings
• String, NMTOKEN, NMTOKENS, NormalizedString
• Others
• Qname, AnyURI, ID, IDREFS, Language, Entity, ...
• In summary, 44 pre-defined simple types
• Question How many does SQL have?

26
Derived SimpleTypes

• Restrict domain
• ltxsdsimpleType nameMyIntegergt
• ltxsdrestriction basexsdintegergt
• ltxsdminInclusive value10000/gt
• ltxsdmaxInclusive value99999/gt
• lt/xsdrestrictiongt
• lt/xsdsimpleTypegt
• minInclusive, maxInclusive are Facets

27
Derived SimpleTypes

• Restriction by Pattern Matching
• Currencies have three capital letters
• ltxsdsimpleType nameCurrencygt
• ltxsdrestriction basexsdstring gt
• ltxsdpattern valueA-Z3/gt
• lt/xsdrestrictiongt
• lt/xsdsimpleTypegt

28
Derived SimpleTypes

• Restriction by Enumeration
• ltxsdsimpleType nameCurrencygt
• ltxsdrestriction basexsdstring gt
• ltxsdenumeration valueATS/gt
• ltxsdenumeration valueEUR/gt
• ltxsdenumeration valueGBP/gt
• ltxsdenumeration valueUSD/gt
• lt/xsdrestrictiongt
• lt/xsdsimpleTypegt

29
Derived SimpleTypes

• There are 15 different kinds of Facets
• e.g., minExclusive, totalDigits, ...
• Most built-in types are derived from other
  built-in types by restriction
• e.g., Integer is derived from Decimal
• there are only 19 base types (out of 44)
• Ref Appendix B of XML Schema Primer

30
List Types

• SimpleType for Lists
• Built-in List Types IDREFS, NMTOKENS
• User-defined List Types
• ltxsdsimpleType name intList gt
• ltxsdlist itemType xsdinteger /gt
• lt/xsdsimpleTypegt
• Items in instances are separed by whitespace
• 5 -10 7 -20
• Facets for Restrictions
• length, minLength, maxLength, enumeration

31
Facets of List Types

• ltxsdsimpleType name Participants gt
• ltxsdlist itemType xsdstring /gt
• ltxsdsimpleTypegt
• ltxsdsimpleType name Medalists gt
• ltxsdrestriction base Participants gt
• ltxsdlength value 3 /gt
• lt/xsdrestrictiongt
• lt/xsdsimpleTypegt

32
Union Types

• Corresponds to the in DTDs
• (Variant Records in Pascal or Union in C)
• Valid instances are valid to any of the types
• ltxsdsimpleType name Potpurri gt
• ltxsdunion memberTypes xsdstring
  intList/gt
• lt/xsdsimpleTypegt
• Valid Instanzes
• fünfzig 1 3 17 wunderbar 15
• Supported Facets
• pattern, enumeration

33
Choice Union in ComplexTypes

• A book has either an author or an editor
• ltxsdcomplexType name Book gt ltxsdsequencegt
• ltxsdchoicegt
• ltxsdelement name author type
  Person
• maxOccurs
  unbounded /gt
• ltxsdelement name editor type
  Person /gt
• lt/xsdchoicegt
• lt/xsdsequencegt lt/xsdcomplexTypegt

34
Element Groups

• If the book has an editor, then the book also
  has a sponsor
• ltxsdcomplexType name Book gt ltxsdsequencegt
• ltxsdchoicegt
• ltxsdelement name Author type Person
  .../gt
• ltxsdgroup ref EditorSponsor /gt
• lt/xsdchoicegt lt/xsdsequencegt lt/xsdcomplexTypegt
• ltxsdgroup name EditorSponsor gt
  ltxsdsequencegt ltxsdelement name Editor
  typePerson /gt
• ltxsdelement name Sponsor type Org /gt
• lt/xsdsequencegt lt/xsdgroupgt

35
Optional Element Groups

• All or nothing unordered content
• PubInfo has name, year, city or gar
  nothing
• ltxsdcomplexType name PubInfo gt
  ltxsdsequencegt
• ltxsdallgt
• ltxsdelement name name type
  xsdstring/gt
• ltxsdelement name year type xsdstring
  /gt
• ltxsdelement name ort type xsdstring
  /gt
• lt/xsdallgt lt!-- Attributdeklarationen --gt
• lt/xsdsequencegt lt/xsdcomplexTypegt
• No other element declarations allowed!!!
• maxOccurs must be 1

36
Attribute Groups

• ltxsdattributeGroup name PriceInfo gt
• ltxsdattribute name curr type
  xsdstring /gt
• ltxsdattribute name val type
  xsddecimal /gt
• lt/xsdattributeGroupgt
• ltxsdcomplexType name Book gt
• ...
• ltxsdattributeGroup ref PriceInfo /gt
• lt/xsdcomplexTypegt

37
Definition of Keys

• Keys are defined as part of elements
• Special sub-element key
• selector describes the context (e.g., emp in a
  DB)
• field describes the key within context
• several fields composite keys
• selector and fields are XPath expressions
• Validation
• Eval selector -gt Sequence of Nodes
• Eval fields on Nodes -gt Set of tuples
• Check that there are no duplicates in Set of
  tuples

38
Definition of Keys

• isbn is key of books in bib
• ltelement name bibgt ltcomplexTypegt ltsequencegt
• ltelement book maxOccurs unboundedgt
  ltcomplexTypegt ltsequencegt ... lt/sequencegt
• ltattribute name isbn type
  string /gt
• lt/complexTypegt lt/elementgt lt/sequencegt
• ltkey name constraintX gt
• ltselector xpath book /gt
  ltfield xpath _at_isbn /gt
• lt/keygt
• lt/complexTypegt lt/elementgt

39
References (foreign keys)

• Part of the definition of an element
• Concept selector and field(s)
• selector determines context of the foreign keys
• field(s) specify the foreign key
• refer gives the scope of the references (key
  constr.)
• Syntax e.g., books referencing other books
• ltkeyref name constraintY refer
  constraintX gt
• ltselector xpath book/references /gt
• ltfield xpath _at_isbn /gt
• lt/keyrefgt

40
UNIQUE Constraints

• Same concept as in SQL
• uniqueness, but no referentiability
• Syntax and concept almost the same as for keys
• ltunique name constraintZgt
• ltselector xpath book /gt
• ltfield xpath title /gt
• lt/uniquegt
• Part of the definition of an element

41
Null Values

• not there vs. unknown (i.e., null)
• empty vs. unknown
• Concept Attribute nilwith value true
• Only works for elements
• Schema definition NULL ALLOWED
• ltxsdelement name publisher type PubInfo
• nillable true /gt
• Valid Instance with content unknown
• ltpublisher xsinil true /gt
• xsi Namespace for predefined Instances
• Publisher may have other attributes, but content
  must be empty!

42
Derived Complex Types

• Two concepts of subtyping / inheritance
• Subtyping via Extension
• Add Elementen
• Similar to inheritance in OO
• Subtyping via Restriction
• e.g., constrain domains of types used
• substituitability is preserved
• Further features
• Constrain Sub-typisierung (final)
• Abstracte Types

43
Subtyping via Extension

• A book is a publikation
• ltxsdcomplexType name Publicationgt
  ltxsdsequencegt
• ltxsdelement name title type
  xsdstring /gt
• ltxsdelement name year type
  xsdinteger /gt
• lt/xsdsequencegt lt/xsdcomplexTypegt
• ltxsdcomplexType name Bookgt
  ltxsdcomplexContentgt
• ltxsdextension base Publication gt
  ltxsdsequencegt
• ltxsdelement name author type
  Person /gt
• lt/xsdsequencegt lt/xsdextensiongt
• lt/xsdcomplexContentgt lt/xsdcomplexTypegt

44
Subtyping by Extension

• A bib contains Publications
• ltxsdelement name bib gt ltxsdsequencegt
• ltxsdelement name pub type Publication
• maxOccurs unbounded/gt
• lt/xsdsequencegt lt/xsdelementgt
• pub Elements may be books!
• Instanzes have xsitype Attribute
• ltbibgt ltpub xsitype Bookgt
• lttitlegtWilde Wutzlt/titlegtltyeargt1984lt/yeargt
• ltauthorgtD.A.K.lt/authorgt lt/pubgt
• lt/bibgt

45
Subtyping via Restriction

• The following restrictions are allowed
• Instances of subtypes have default values
• Instances of subtypes are fixed (i.e., constant)
• Instances of subtypes have stronger types (e.g.,
  string vs. anyType)
• Instances of subtypes have mandatory fields which
  optional in supertype
• Supertype.minOccurs lt Subtype.minOccursSupertype
  .maxOccurs gt Subtype.maxOccurs

46
Subtyping via Restriction

• ltcomplexType name superTypegt ltsequencegt
• ltelement name a type string minOccurs
  0 /gt
• ltelement name b type anyType /gt
• ltelement name c type decimal /gt
• lt/sequencegt ltcomplexTypegt
• ltcomplexType name subTypegt ltcomplexContentgt
• ltrestriction base superTypegt ltsequencegt
• ltelement name a type string
  minOccurs 0
  maxOccurs 0 /gt
• ltelement name b type string /gt
• ltelement name c type decimal /gt
• lt/sequencegt lt/restrictiongt
• lt/complexContentgt lt/complexTypegt

47
Substitution Groups

• Elements, which substitute global elem.
• E.g., editor is a personltelement name
  person type string /gtltcomplexType name
  Book gt ltsequencegt ltelement ref person /gt
  ...lt/sequencegt lt/complexTypegtltelement name
  author type string
  substitutionGroup person /gtltelement name
  editor type string
  substitutionGroup person /gt

48
Abstract Elements and Types

• No instances exist
• Only instances of subtypes of substitions exist
• person in Book must be an author or editor
  ltelement name person type string
  abstract true /gtltcomplexType name
  Book gt ltsequencegt ltelement ref person
  /gt ...lt/sequencegt lt/complexTypegt...

49
Constrain Subtyping

• Corresponds to final in Java
• XML Schema is more clever!(?)
• Constrain the kind of subtyping (extension,
  restriction, all)
• Constrain the facets used
• ltsimpleType name ZipCode gt
• ltrestriction base stringgt
• ltlength value 5 fixed true /gt
• lt/restrictiongt ltsimpleTypegt
• ltcomplexType name Book final restriction
  gt
• ... lt/complexTypegt
• You may subtype ZipCode. But all subtypes have
  length 5.

50
Constrain Substituability

• ltcomplexType name Book block all gt
• ... lt/complexTypegt
• It is possible to define subtypes of Book
• So, it is possible to reuse structe of Book
• But instances of subtypes of Book are NOT books
  themselves.
• (Now, things get really strange!)

51
Namespaces and XML Schema

• Declare the Namespace of Elements?
• TargetNamespace for Global Elements
• qualifies names of root elements
• elementFormDefault
• qualifies names of local (sub-) elements
• attributeFormDefault
• qualifies names of attributes

52
Namespaces in the Schemadef.

• ltxsdschema xmlnsxsdhttp//w3.org/2001/XMLSche
  ma
• xmlnsbohttp//www.Book.com
• targetNamespacehttp//www.Book.c
  omgt
• ltxsdelement namebook typeboBookType/gt
• ltxsdcomplexType nameboBookType gt
• ... lt/xsdcomplexTypegt
• lt/xsdschemagt
• book und BookType are part of TargetNameSp.

53
Namespaces in Schemadef.

• ltschema xmlns http//w3.org/2001/XMLSchema
• xmlnsbohttp//www.Book.com
• targetNamespacehttp//www.Book.c
  om gt
• ltelement namebook type boBookType /gt
• ltcomplexType nameBookType gt
• ... lt/complexTypegt
• lt/schemagt

54
Namespaces in Schemadef.

• ltxsdschema xmlnsxsdhttp//w3.org/2001/XMLSche
  ma
• xmlns http//www.Book.com
  
• targetNamespacehttp//www
  .Book.com gt
• ltxsdelement namebook type BookType /gt
• ltxsdcomplexType xsdnameBookType gt
• ... lt/xsdcomplexTypegt
• lt/xsdschemagt
• Target www.Book.com as Default Namespace

55
Instances of www.Book.com

• ltbobook xmlnsbo http//www.Book.com gt
• ...
• lt/bobookgt
• Valid according to all three schemas!

56
Schema Location in Instance

• Declare within an XML document, where to find the
  schema that should valid that document
• Declare target Namespace
• Declare URI of Schema
• ltbook xmlns http//www.Book.com
• xmlnsxsi http//w3.org/XMLSchema-instance
• xsischemaLocation http//www.Book.com
• http//www.book.com/Book.xsd
• ...
• lt/bookgt
• This is not enforced! Validation using other
  Schemas is legal.

57
Unqualified Locals

• Local Declarations are not qualifiziert
• ltbobook xmlnsbo http//www.Book.com
• price 69.95 curr EUR gt
• lttitlegtDie wilde Wutzlt/titlegt ...
• lt/bobookgt
• Valide Instance globally qualifed, locally not
• Even works within Schema
• ltxsdelement name ... type ... /gt
• Full flexibility to control use of namespaces

58
Qualifizied Sub-elements

• ltschema xmlns http//w3.org/2001/XMLSchema
• xmlnsbohttp//www.Book.com
• targetNamespacehttp//www.Book.c
  om gt
• elementFormDefaultqualified
• ltelement namebook type boBookType /gt
• ltcomplexType nameBookType gt ltsequencegt
• ltelement name title type string /gt
• ltelement name author /gt ltsequencegt
• ltelement name vname type string
  /gt
• ltelement name nname type string
  /gt
• lt/sequencegt lt/sequencegt lt/complexTypegt
• lt/schemagt

59
Valid Instances

• ltbobook xmlnsbo http//www.Book.com
  ltbotitlegtDie wilde Wutzlt/botitlegt
• ltboauthorgtltbovnamegtD.lt/bovnamegt
  ltbonnamegtK.lt/bonnamegtlt/boauthorgt
• lt/bobookgt
• ltbook xmlns http//www.Book.com lttitlegtDie
  wilde Wutzlt/titlegt
• ltauthorgtltvnamegtD.lt/vnamegt
  ltnnamegtK.lt/nnamegtlt/authorgt
• lt/bookgt

60
Qualified Attributes

• Enforce Qualified Attributes
• attributeFormDefault qualified in Element
  definition
• Enforce that certain attributes must be qualified
• ltattribute name ... type ... form
  qualified /gt
• (Analogous, enforce that Sub-elements must be
  qualified)

61
Composition of Schemas

• Construct libraries of schemas
• Include a Schema
• Parent and child have the same Target Namespace
• Only Parent used for Validation
• Redefine Include Modify
• Again, parent and child have the same Target
  Namespace
• Include individual types from a schema
• ltelement ref libimpType /gt

62
Summary

• XML Schema is very powerful
• simple Types and complex Types
• many pre-defined types
• many ways to derive and create new types
• adopts database concepts (key, foreign keys)
• full control and flexibility
• fully inline with namespaces and other XML
  standards
• XML Schema is too powerful?
• too complicated, confusing?
• difficult to implement
• people use only a fraction anyway
• XML Schema is very different to what you know!
• the devil is in the detail

63
XML vs. OO

• Encapsulation
• OO hides data
• XML makes data explicit
• Type Hierarchy
• OO defines superset / subset relationship
• XML shares structure set rel. make no sense
• Data Behavior
• OO packages them together
• XML separates data from its interpretation

64
XML vs. Relational

• Structural Differences
• Tree vs. Table
• Heterogeneous vs. Homegeneous
• Optional vs. Strict typing
• Unnormalized vs. Normalized data
• Some commonalities
• Logical and physical data independance
• Declarative semantics
• Generic data model