XML

1
XML
2
Introduction

• XML Extensible Markup Language
• Defined by the WWW Consortium (W3C)
• Originally intended as a document markup language
  not a database language
• Documents have tags giving extra information
  about sections of the document
• E.g. lttitlegt XML lt/titlegt ltslidegt Introduction
  lt/slidegt
• Derived from SGML (Standard Generalized Markup
  Language), but simpler to use than SGML
• Extensible, unlike HTML
• Users can add new tags, and separately specify
  how the tag should be handled for display
• Goal was (is?) to replace HTML as the language
  for publishing documents on the Web

3
XML Introduction (Cont.)

• The ability to specify new tags, and to create
  nested tag structures made XML a great way to
  exchange data, not just documents.
• Much of the use of XML has been in data exchange
  applications, not as a replacement for HTML
• Tags make data (relatively) self-documenting
• ltbankgt
• ltaccountgt
• ltaccount-numbergt A-101
  lt/account-numbergt
• ltbranch-namegt Downtown
  lt/branch-namegt
• ltbalancegt 500
  lt/balancegt
• lt/accountgt
• ltdepositorgt
• ltaccount-numbergt A-101
  lt/account-numbergt
• ltcustomer-namegt Johnson
  lt/customer-namegt
• lt/depositorgt
• lt/bankgt

4
XML Motivation

• Data interchange is critical in todays networked
  world
• Examples
• Banking funds transfer
• Order processing (especially inter-company
  orders)
• Scientific data
• Chemistry ChemML,
• Genetics BSML (Bio-Sequence Markup Language),
  
• Paper flow of information between organizations
  is being replaced by electronic flow of
  information
• Each application area has its own set of
  standards for representing information
• XML has become the basis for all new generation
  data interchange formats

5
XML Motivation (Cont.)

• Earlier generation formats were based on plain
  text with line headers indicating the meaning of
  fields
• Similar in concept to email headers
• Does not allow for nested structures, no standard
  type language
• Tied too closely to low level document structure
  (lines, spaces, etc)
• Each XML based standard defines what are valid
  elements, using
• XML type specification languages to specify the
  syntax
• DTD (Document Type Descriptors)
• XML Schema
• Plus textual descriptions of the semantics
• XML allows new tags to be defined as required
• However, this may be constrained by DTDs
• A wide variety of tools is available for parsing,
  browsing and querying XML documents/data

6
Structure of XML Data

• Tag label for a section of data
• Element section of data beginning with lttagnamegt
  and ending with matching lt/tagnamegt
• Elements must be properly nested
• Proper nesting
• ltaccountgt ltbalancegt . lt/balancegt lt/accountgt
• Improper nesting
• ltaccountgt ltbalancegt . lt/accountgt lt/balancegt
• Formally every start tag must have a unique
  matching end tag, that is in the context of the
  same parent element.
• Every document must have a single top-level
  element

7
Example of Nested Elements

• ltbank-1gt ltcustomergt
• ltcustomer-namegt Hayes lt/customer-namegt
• ltcustomer-streetgt Main lt/customer-streetgt
• ltcustomer-citygt Harrison
  lt/customer-citygt
• ltaccountgt
• ltaccount-numbergt A-102 lt/account-numbergt
• ltbranch-namegt Perryridge
  lt/branch-namegt
• ltbalancegt 400 lt/balancegt
• lt/accountgt
• ltaccountgt
• lt/accountgt
• lt/customergt . .
• lt/bank-1gt

8
Motivation for Nesting

• Nesting of data is useful in data transfer
• Example elements representing customer-id,
  customer name, and address nested within an order
  element
• Nesting is not supported, or discouraged, in
  relational databases
• With multiple orders, customer name and address
  are stored redundantly
• normalization replaces nested structures in each
  order by foreign key into table storing customer
  name and address information
• Nesting is supported in object-relational
  databases
• But nesting is appropriate when transferring data
• External application does not have direct access
  to data referenced by a foreign key

9
Structure of XML Data (Cont.)

• Mixture of text with sub-elements is legal in
  XML.
• Example
• ltaccountgt
• This account is seldom used any more.
• ltaccount-numbergt A-102lt/account-numbergt
• ltbranch-namegt Perryridgelt/branch-namegt
• ltbalancegt400 lt/balancegtlt/accountgt
• Useful for document markup, but discouraged for
  data representation

10
Attributes

• Elements can have attributes
• ltaccount acct-type checking gt
• ltaccount-numbergt A-102
  lt/account-numbergt
• ltbranch-namegt Perryridge
  lt/branch-namegt
• ltbalancegt 400 lt/balancegt
• lt/accountgt
• Attributes are specified by namevalue pairs
  inside the starting tag of an element
• An element may have several attributes, but each
  attribute name can only occur once
• ltaccount acct-type checking monthly-fee5gt

11
Attributes Vs. Subelements

• Distinction between subelement and attribute
• In the context of documents, attributes are part
  of markup, while subelement contents are part of
  the basic document contents
• In the context of data representation, the
  difference is unclear and may be confusing
• Same information can be represented in two ways
• ltaccount account-number A-101gt .
  lt/accountgt
• ltaccountgt ltaccount-numbergtA-101lt/account-numb
  ergt lt/accountgt
• Suggestion use attributes for identifiers of
  elements, and use subelements for contents

12
More on XML Syntax

• Elements without subelements or text content can
  be abbreviated by ending the start tag with a /gt
  and deleting the end tag
• ltaccount numberA-101 branchPerryridge
  balance200 /gt
• To store string data that may contain tags,
  without the tags being interpreted as
  subelements, use CDATA as below
• lt!CDATAltaccountgt lt/accountgtgt
• Here, ltaccountgt and lt/accountgt are treated as
  just strings

13
Namespaces

• XML data has to be exchanged between
  organizations
• Same tag name may have different meaning in
  different organizations, causing confusion on
  exchanged documents
• Specifying a unique string as an element name
  avoids confusion
• Better solution use unique-nameelement-name
• Avoid using long unique names all over document
  by using XML Namespaces
• ltbank XmlnsFBhttp//www.FirstBank.comgt
  
• ltFBbranchgt
• ltFBbranchnamegtDowntownlt/FBbranchnamegt
• ltFBbranchcitygt Brooklyn lt/FBbranchcitygt
• lt/FBbranchgt
• lt/bankgt

14
XML Document Schema

• Database schemas constrain what information can
  be stored, and the data types of stored values
• XML documents are not required to have an
  associated schema
• However, schemas are very important for XML data
  exchange
• Otherwise, a site cannot automatically interpret
  data received from another site
• Two mechanisms for specifying XML schema
• Document Type Definition (DTD)
• Widely used
• XML Schema
• Newer, increasing use

15
Document Type Definition (DTD)

• The type of an XML document can be specified
  using a DTD
• DTD constraints structure of XML data
• What elements can occur
• What attributes can/must an element have
• What subelements can/must occur inside each
  element, and how many times.
• DTD does not constrain data types
• All values represented as strings in XML
• DTD syntax
• lt!ELEMENT element (subelements-specification) gt
• lt!ATTLIST element (attributes) gt

16
Element Specification in DTD

• Subelements can be specified as
• names of elements, or
• PCDATA (parsed character data), i.e., character
  strings
• EMPTY (no subelements) or ANY (anything can be a
  subelement)
• Example
• lt! ELEMENT depositor (customer-name
  account-number)gt
• lt! ELEMENT customer-name (PCDATA)gt
• lt! ELEMENT account-number (PCDATA)gt
• Subelement specification may have regular
  expressions
• lt!ELEMENT bank ( ( account customer
  depositor))gt
• Notation
• - alternatives
• - 1 or more occurrences
• - 0 or more occurrences

17
Bank DTD

• lt!DOCTYPE bank
• lt!ELEMENT bank ( ( account customer
  depositor))gt
• lt!ELEMENT account (account-number branch-name
  balance)gt
• lt! ELEMENT customer(customer-name
  customer-street
  
  customer-city)gt
• lt! ELEMENT depositor (customer-name
  account-number)gt
• lt! ELEMENT account-number (PCDATA)gt
• lt! ELEMENT branch-name (PCDATA)gt
• lt! ELEMENT balance(PCDATA)gt
• lt! ELEMENT customer-name(PCDATA)gt
• lt! ELEMENT customer-street(PCDATA)gt
• lt! ELEMENT customer-city(PCDATA)gt
• gt

18
Attribute Specification in DTD

• Attribute specification for each attribute
• Name
• Type of attribute
• CDATA
• ID (identifier) or IDREF (ID reference) or IDREFS
  (multiple IDREFs)
• more on this later
• Whether
• mandatory (REQUIRED)
• has a default value (value),
• or neither (IMPLIED)
• Examples
• lt!ATTLIST account acct-type CDATA checkinggt
• lt!ATTLIST customer
• customer-id ID REQUIRED
• accounts IDREFS REQUIRED gt

19
IDs and IDREFs

• An element can have at most one attribute of type
  ID
• The ID attribute value of each element in an XML
  document must be distinct
• Thus the ID attribute value is an object
  identifier
• An attribute of type IDREF must contain the ID
  value of an element in the same document
• An attribute of type IDREFS contains a set of (0
  or more) ID values. Each ID value must contain
  the ID value of an element in the same document

20
Bank DTD with Attributes

• Bank DTD with ID and IDREF attribute types.
• lt!DOCTYPE bank-2
• lt!ELEMENT account (branch, balance)gt
• lt!ATTLIST account
• account-number ID
  REQUIRED
• owners IDREFS
  REQUIREDgt
• lt!ELEMENT customer(customer-name,
  customer-street,
• 
  customer-city)gt
• lt!ATTLIST customer
• customer-id ID
  REQUIRED
• accounts IDREFS
  REQUIREDgt
• declarations for branch, balance,
  customer-name,
  customer-street and customer-citygt

21
XML data with ID and IDREF attributes

• ltbank-2gt
• ltaccount account-numberA-401 ownersC100
  C102gt
• ltbranch-namegt Downtown lt/branch-namegt
• ltbalancegt 500 lt/balancegt
• lt/accountgt
• ltcustomer customer-idC100 accountsA-401gt
• ltcustomer-namegtJoe
  lt/customer-namegt
• ltcustomer-streetgt Monroe
  lt/customer-streetgt
• ltcustomer-citygt Madisonlt/customer-ci
  tygt
• lt/customergt
• ltcustomer customer-idC102 accountsA-401
  A-402gt
• ltcustomer-namegt Mary
  lt/customer-namegt
• ltcustomer-streetgt Erin
  lt/customer-streetgt
• ltcustomer-citygt Newark
  lt/customer-citygt
• lt/customergt
• lt/bank-2gt

22
XML Schema

• XML Schema is a more sophisticated schema
  language which addresses the drawbacks of DTDs.
  Supports
• Typing of values
• E.g. integer, string, etc
• Also, constraints on min/max values
• User defined types
• Is itself specified in XML syntax, unlike DTDs
• More standard representation, but verbose
• Is integrated with namespaces
• Many more features
• List types, uniqueness and foreign key
  constraints, inheritance ..
• BUT significantly more complicated than DTDs,
  not yet widely used.

23
Querying and Transforming XML Data

• Translation of information from one XML schema to
  another
• Querying on XML data
• Above two are closely related, and handled by the
  same tools
• Standard XML querying/translation languages
• XPath
• Simple language consisting of path expressions
• XSLT
• Simple language designed for translation from XML
  to XML and XML to HTML
• XQuery
• An XML query language with a rich set of features
• Wide variety of other languages have been
  proposed, and some served as basis for the Xquery
  standard
• XML-QL, Quilt, XQL,

24
Tree Model of XML Data

• Query and transformation languages are based on a
  tree model of XML data
• An XML document is modeled as a tree, with nodes
  corresponding to elements and attributes
• Element nodes have children nodes, which can be
  attributes or subelements
• Text in an element is modeled as a text node
  child of the element
• Children of a node are ordered according to their
  order in the XML document
• Element and attribute nodes (except for the root
  node) have a single parent, which is an element
  node
• The root node has a single child, which is the
  root element of the document
• We use the terminology of nodes, children,
  parent, siblings, ancestor, descendant, etc.,
  which should be interpreted in the above tree
  model of XML data.

25
XPATH

• XPath is used to address (select) parts of
  documents using path expressions
• A path expression is a sequence of steps
  separated by /
• Think of file names in a directory hierarchy
• Result of path expression set of values that
  along with their containing elements/attributes
  match the specified path

26
XPATH

• E.g. /regionSet/region/COUNTRIES/COUNTRIES_I
  TEM/COUNTRY_NAME evaluated on the regions
  returns
•  
• ltCOUNTRY_NAMEgtSudanlt/COUNTRY_NAMEgt
• ltCOUNTRY_NAMEgtSwazilandlt/COUNTRY_NAMEgt
•  
•   ltCOUNTRY_NAMEgtSouth Georgia and the South
  Sandwich Islandslt/COUNTRY_NAMEgt
•  

27
XPath Example

• Sample XML
• The root element
• /STATES
• The SCODE of all STATE elements of STATES element
  
• /STATES/STATE/SCODE
• All the CAPTIAL element with a CNAME sub-element
  of the STATE element of the STATES element
• /STATES/STATE/CAPITALCNAMEAtlanta
• All CITIES elements in the XML document
• //CITIES

28
More XPath Example

• Element AA with two ancestors
• ///AA
• First BB element of AA element
• /AA/BB1
• All the CC elements of the BB elements which has
  an sub-element A with value 3
• /BBA3/CC
• Any elements AA or elements CC of elements BB
• //AA /BB/CC

29
Even More XPath Example

• Select all sub-elements of elements BB of
  elements AA
• /BB/AA/
• When you do not know the sub-elements
• Different from /BB/AA
• Select all attributes named aa
• //_at_aa
• Select all CITIES elements with an attribute
  named aa
• //CITIES_at_aa
• Select all CITIES elements with an attribute
  named aa with value 123
• //CITIES_at_aa 123

30
Axis

• Context node
• Evaluation of XPath is from left to right
• The context node the current node (set) being
  evaluated
• Axis
• Specifies the relationship of the resulting
  nodes relative to context node
• Example
• /childAA children of AA, abbreviated by /AA
• //AA/ancestorBB BB elements who are ancestor
  of any AA elements

31
Axes

• ancestor //BBB/ancestor
•  ltAAAgt           ltBBB/gt           ltCCC/gt
            ltBBB/gt           ltBBB/gt
            ltDDDgt                ltBBB/gt
            lt/DDDgt           ltCCC/gt   lt/AAAgt

32
Axes

• ancestor //BBB/ancestorDDD
•  ltAAAgt           ltBBB/gt           ltCCC/gt
            ltBBB/gt           ltBBB/gt
            ltDDDgt                ltBBB/gt
            lt/DDDgt           ltCCC/gt   lt/AAAgt

33
Axes

• attribute Contains all attributes of the current
  node
• //BBB/attribute abbreviated by //_at_
• ltAAAgt           ltBBB aa1/gt           ltCCC/gt
            ltBBB aa2 /gt           ltBBB aa3
  /gt           ltDDDgt                ltBBB bb31
  /gt           lt/DDDgt           ltCCC/gt   lt/AAAgt
• //BBB/attributebb

34
Axes

• child
• /AAA/DDD/childBBB child can be omitted for
  abbreviation
•  ltAAAgt           ltBBB/gt           ltCCC/gt
            ltBBB/gt           ltBBB/gt
            ltDDDgt                ltBBB/gt
            lt/DDDgt           ltCCC/gt   lt/AAAgt

35
Axes

• descendant
• /AAA/descendent
• ltAAAgt           ltBBB/gt           ltCCC/gt
            ltBBB/gt           ltBBB/gt
            ltDDDgt                ltBBB/gt
            lt/DDDgt           ltCCC/gt   lt/AAAgt
• /AAA/descendentCCC ?

36
Axes

• parent
• //BBB/parent
• ltAAAgt           ltBBB/gt           ltCCC/gt
            ltBBB/gt           ltBBB/gt
            ltDDDgt                ltBBB/gt
            lt/DDDgt           lt CCC/gt   lt/AAAgt
• //BBB/parentDDD ?

37
Axes

• descendant-or-self
• following
• following-sibling
• preceding
• preceding-sibling
• self

38
Predicates

• Filters a element set
• A predicate is placed inside square brackets (
  )
• Example //BBBposition() mod 2 0
•    ltAAAgt           ltBBB/gt           ltBBB/gt
            ltBBB/gt           ltBBB/gt
            ltBBB/gt           ltBBB/gt
            ltBBB/gt           ltBBB/gt
            ltCCC/gt           ltCCC/gt
            ltCCC/gt      lt/AAAgt

39
Predicates

• //BBB_at_aa31
• ltAAAgt           ltBBB aa1/gt           ltCCC/gt
            ltBBB aa2 /gt           ltBBB aa3
  /gt           ltDDDgt                ltBBB bb31
  /gt           lt/DDDgt           ltCCC/gt   lt/AAAgt
• Is it different from //BBB/attributebb?