Semistructured Data and XML

1
Semistructured Data and XML

• IntroductionChapter 30.1-30.2.2, but 30.1.2

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Semistructured Data

• Data that does not conform to a fixed schema
• Has some structure, but the structure is not
  rigid
• Reasons for that
• Structure of data is unknown
• Data come from different sources with different
  structures

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Semistructured Data Management

• DBMS
• Schema is known a priori
• Query language is declarative
• Semistructured data
• Tree-labeled representation
• Query language is navigational

4
Object Exchange Model

• Nested object model
• OEM object can be
• Atomic contains a value of a base type
• Complex contains a complex value a collection
  of OEM objects (subobjects)
• Complex object is a parent, a subobject is a
  child
• OEM object can have several parent objects

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OEM Objects

• Each object is described by quadruple
• Label string of characters describing what
  object represents
• Identifying object
• Identifying the meaning of the object
• Type data type of the object value
• Value variable length object value
• Object-ID unique identifier used by the system
  and not presented to users

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Diagrammatic representation of OEM

• Directed graph
• Nodes objects
• Edges denote connections of objects and
  subobjects
• Atomic object has no outgoing edge
• Complex object has outgoing edges

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OEM vs. OO Data Model

• OEM is information exchange model
• Does not specify how object is stored
• Specifies how object is received
• OEM supports
• Object nesting
• Object identity
• OEM does not support classes, inheritance,
  methods and polymorphism

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XML

• Extensible means it can be used to create lots of
  document types
• Markup means it specifies the structure and the
  content of a document
• Language this is computer language

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XML vs. HTML

• Common features
• Parent SGML
• Text-based
• Building blocks are tags, attributes and elements
• HTML language for data presentation and
  formatting
• No mechanism for monitoring documents content
• XML emphasizes data content

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Basic Terms

• XML meta language
• XML application
• vocabulary
• XML parser reads XML documents and provides
  access to their content and structure
• Fatal error

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Basic Terms (cont)

• Well-formed document contains no syntax errors
  and satisfies W3C specifications
• Valid document well-formed document that
  satisfied the rules of a particular XML
  application

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Document Structure

• Logical structure a set of elements or
  containers
• An elements type (generic identifier)
  constraints element content
• Name
• Set of attributes
• Attribute is specified as a pair name value

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Main Document Components

• The prolog
• The root element
• Other elements

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Documents Prolog

• XML declaration
• Processing instructions
• Document type declaration
• Comments
• White space

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XML Declaration

• First line of the document
• lt?xml version1.0 encodingUTF-8
• standaloneyes?gt
• Version 1.0 corresponds to the W3C XML
  Recommendation 1.0
• Encoding UTF-8 - default, Unicode, UCS-2,
  UCS-4,
• Standalone references to external files

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Processing Instructions

• The instructions which are passed by XML parser
  to the application
• Syntax
• lt?piname pidata?gt
• Can be used to connect XML documents with CSS
• lt?xml-stylesheet typetext/css hrefURL?gt

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Elements

• An element start and end tags
• ltpricegt 1987.47lt/pricegt
• Empty element an element with no data
• lthead/gt
• Nesting elements
• ltbookgt
• lttitlegt Database Systems lt/titlegt
• ltauthorgt Connolly, Begg lt/authorgt
• lt/bookgt

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Attributes

• Attribute name conforms the following
• Begins with a letter or _
• Unique within the tag
• Doesnt begin with xml
• Space is not allowed
• Attribute value is the information processed by
  the parser but is not viewed

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Attribute Example

• ltbook categoryITgt
• lttitlegt Database Systems lt/titlegt
• ltauthorgt Connolly, Begg lt/authorgt
• lt/bookgt

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Displaying XML Documents

• Web browsers have XML parsers
• Only well-formed documents are displayed
• With style sheet
• Without style sheet
• Displaying errors

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Data Binding

• Mapping and synchronization of data in XML
  documents to HTML document
• Data source
• Data consumer
• Involves
• Moving data to a local system
• Manipulating the data

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Data Binding with Internet Explorer

• Data binding agent and table repetition agent are
  utilities implemented as DLL in Internet Explorer
• Data Source Objects
• Tabular data control
• Remote data service
• JDBC DataSource applet
• XML data source object

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XML Data Source Object

• Gathers data from different XML documents
• Has unique methods to manipulate the data
• Supports only well-formed and valid XML documents
• Uses ADO to access the data
• We can apply methods supported by ADO to data
  source object

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Data Source Structure

• A Field a single item of information
• A record a collection of fields
• Pseudo field Text contains character data from
  all the fields in a record
• A recordset collection of records
• Simple recordset
• Hierarchical recordset

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Recordsets

• Simple recordset
• Single root element
• Each record has the same number of fields
• Each field contains character data only
• Hierarchical recordset
• A collection of records nested inside of each
  other

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Data Consumer Elements

• Elements used to display XML data
• lta hrefgt
• ltdivgt
• ltspangt
• ltbuttongt
• ltinput typegt
• lttablegt
• And some other

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Data Islands

• Source of data which is displayed
• XML code describing the data
• Internal data island
• Is placed within the BODY of HTML document
• External data island XML document

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Internal Data Island

• ltbodygt
• ltxml idisland_namegt
• lt?xml version1.0 ?gt
• XML code
• lt/xmlgt
• lt/bodygt

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External Islands

• ltbodygt
• ltxml id island_name
• srcURL_of XML_ filegt lt/xmlgt
• .
• lt/bodygt

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Binding a Single-Valued Data Consumer to XML Data

• Create a data island with the name ILN
• Within the body part of HTML document place a tag
  with attributes
• ltdiv datasrcILN datafldfieldgtlt/divgt
• where field is XML element or attribute

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Working with Simple Recordset

• Is based on ADO recordset
• Recordset corresponds to a specified data island
• Calling recordset methods
• id.recordset.method()
• Calling recorset properties
• id.recordset.property

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Recordset Methods

• Move to the first record in the recordset
• id.recordset.moveFirst()
• Move to the previous record
• id.recordset.movePrevious()
• Move to the next record
• id.recordset.moveNext()
• Move to the last record in the recordset
• id.recordset.moveLast()

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Recordset Properties

• Returns True if the beginning of the recordset is
  reached
• id.recordset.BOF
• Returns True if the end of the recordset is
  reached
• id.recordset.EOF
• Returns the number of current pointer position in
  the recordset
• id.recordset.index
• Returns the number of records in the recordset
• id.recordset.RecordCount

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Add Navigation Buttons with Simple Javascript

• ltbutton onClickid.recordset.moveFirst()"gt
  Firstlt/buttongt
• ltbutton onClickid.recordset.moveNext() if
  (id.recordset.EOF) id.recordset.moveLast()gtNext
  lt/buttongt

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Binding Data to Tables

• Simple recordset
• lttable datasrcidgt
• lttrgt
• lttdgtltspan datafldfield1gtlt/spangtlt/tdgt
• lttdgt
• ltspan datafldfield2gtlt/spangtlt/tdgt
• lt/trgt
• lt/tablegt

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Hierarchical Recordset (example)
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Hierarchical Recorsets

• Two-level recordset
• lttable datasrcid datafldelementgt
• lttrgt
• lttdgtltspan datafldfield1gtlt/spangtlt/tdgt
• lttdgtltspan datafldfield2gtlt/spangtlt/tdgt
• lt/trgt
• lt/tablegt

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Hierarchical Recorsets

• Example
• lttable datasrcid datafldCardgt
• lttrgt
• lttdgtltspan datafldNamegtlt/spangtlt/tdgt
• lttdgtltspan datafldKindgt lt/spangtlt/tdgt
• lt/trgt
• lt/tablegt