CSE 3241: XML Extensible Markup Language (Ch. 12)

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CSE 3241: XML

Extensible Markup Language

(Ch. 12)

1

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Topics



Structured, Semistructured, and Unstructured Data



XML Hierarchical (Tree) Data Model



XML Documents, DTD



XML Schema



Storing and Extracting XML Documents from Databases



XML Languages

(3)

Structured, Semistructured, and Unstructured Data



Structured data

◦ Represented in a strict format

◦ Example: information stored in databases



Semistructured data

◦ Has a certain structure

◦ Not all information collected will have identical structure



Unstructured data

◦ Limited indication of the of data

document that contains information

embedded within it

(4)

What is XML?



XML provides a framework to define a structure for data

◦ An XML document is a collection of related data items

◦ Document is “marked up” with tags known as elements

 Elements are used to provide structure to the data

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XML Hierarchical (Tree) Data Model



Elements and attributes

◦ Main structuring concepts used to construct an XML document



Complex elements

◦ Constructed from other elements hierarchically



Simple elements

◦ Contain data values



XML tag names

◦ Describe the meaning of the data elements in the document

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XML Hierarchical (Tree) Data Model (cont’d.)



XML attributes

◦ Describe properties and

characteristics of the elements (tags) within which they appear

◦ May reference another element in another part of the XML document

 Common to use attribute values in one element as the references

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The XML Data Model



Attributes vs. Elements

◦ Data can be stored in an XML document

either as the contents of an element OR as an attribute of an element

◦ Why pick one over the other?

◦ Best practice:

 Use attributes for information that describes/modifies the element

 Use element contents to hold the actual data values

 Much like in HTML:

 Element (tag) contents are the data to be displayed

 Attributes (generally) modify/describe how it is to be displayed

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XML Document Types – Data Centric XML



Data-centric XML

◦ Highly structured

◦ Many small data items

◦ Often used for data exchange purposes

 Transfer data from one system to another

◦ Also used to create web pages dynamically from databases

◦ Generally follow a schema document that determines their structure

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XML Document Types – Document-Centric XML



Few structural elements



Large amounts of text

◦ Articles, blog entries, books



May have a schema document, but not required

◦ Schema may be very limited in semantics

 What’s a title?

 What’s a chapter?

 What’s a paragraph?

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Hybrid XML

◦ Some parts are highly structured

◦ Some parts mostly blocks of text and/or unstructured

◦ May or may not have a predefined schema



Schemaless XML documents

◦ Semi-structured documents without a predefined schema

◦ Denoted by the attribute

‘standalone=“yes”’ in the XML declaration on the top line

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Valid XML



An XML document is considered valid if:

◦ It is well-formed* , and …

◦ It follows a particular schema in a standard definition language

 A DTD document (Document Type Definition)

 DTDs are the original, older technology

 An XML schema document

 XML schema documents are the “new” hotness

 First published in 2001

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*Well-formed XML



An XML document is well-formed when it follows certain conditions:

◦ It must start with an XML declaration line:

<?xml version=“1.0” standalone=“yes”?>

◦ It must form a tree:

 Must start with a single root element

 Every child element must have start and end tags that are contained completely within a parent

element:

Good Bad

</child> </parent>

</parent> </child>

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DTD – Document Type Definition



Original method of specifying a schema definition

◦ Still in widespread use



A very simple schema definition language

◦ Each possible element in the document is defined

 What children must it have?

 What children can it (optionally) have?

 What kinds of attributes can/must it have?

 If it is a leaf element, what kinds of values

can it have? ¹³

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XML Documents, DTD, and XML Schema (cont’d.)



Notation for specifying elements



XML DTD

◦ Special syntax

 Requires specialized processors

◦ All DTD elements always forced to follow the specified ordering of the document

 Unordered elements not permitted

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A sample XML document and DTD

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<?xml version=“1.0” standalone=“no”?>

<!DOCTYPE Projects SYSTEM “proj.dtd”>

<Name>Product X</Name>

<Location>Bellaire</Bellaire>

<Dept_no>5</Dept_no>

<Last_name>Smith</LastName>

</Worker>

</Worker>

</Workers>

</Project>

…

</Projects>

<!ELEMENT Projects (Project+)>

<!ELEMENT Project (Name, Location, Dept_no?, Workers)>

<!ATTLIST Project number ID

#REQUIRED>

<!ELEMENT Name (#PCDATA)>

<!ELEMENT Location (#PCDATA)>

<!ELEMENT Dept_no (#PCDATA)>

<!ELEMENT Workers (Worker*)>

<!ELEMENT Worker (Ssn, Last_name?, First_name?, Hours)>

<!ELEMENT Ssn (#PCDATA)>

<!ELEMENT Last_name (#PCDATA)>

<!ELEMENT First_name (#PCDATA)>

<!ELEMENT Hours (#PCDATA)>

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XML Document Types - Schemas



Semi-structured data



Defines

◦ names for expected XML tags/elements

◦ required/optional elements

◦ Nested elements

 Defines the entire structure of the documents

 What is the root element?

 What are the possible children of the root element?

 What attributes must/can an element have?



Communicate semantic information about an XML document

◦ What do each of these elements mean? ₁₆

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XML Schema



XML schema definition for the COMPANY database

◦ XML Schema definition includes:

 Strong types (integer, floats, strings, dates, etc.)

 Minimum and maximum occurrences of elements

 Keys! References from one element to another with the xsd:keyref tag

 Breaks the strict hierarchy of XML

 … and many other features

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XML Schema



XML schema language

◦ Standard for specifying the structure of XML documents

◦ Uses same syntax rules as regular XML documents

 Same processors can be used on both

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XML Schema documents



Provide a way to specify an XML schema using XML as a language

◦ Overcomes the limitations of DTD

◦ However, XML schemas are much more complex than DTDs



XML schema a hierarchical, tree model

◦ Makes sense – it’s XML!



Adds concepts from database models

◦ Keys, references, identifiers

²⁰

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XML document and XML Schema

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<?xml version="1.0" encoding="UTF-8"?>

<pwk:Projects

xmlns:pwk="http://www.example.org/ProjectWorkers

"

xmlns:xsi="http://www.w3.org/2001/XMLSchema- instance"

xsi:schemaLocation="http://www.example.org/Proje ctWorkers ../Schemas/ProjectWorkers.xsd ">

<pwk:Project pwk:number="1">

<pwk:Name>Product X</pwk:Name>

<pwk:Location>Bellaire</pwk:Location>

<pwk:Dept_no>1</pwk:Dept_no>

<pwk:Workers>

<pwk:Worker>

<pwk:Ssn>123456789</pwk:Ssn>

<pwk:Last_name>Smith</pwk:Last_name>

<pwk:Hours>32.5</pwk:Hours>

</pwk:Worker>

<pwk:Worker>

<pwk:Ssn>453453453</pwk:Ssn>

<pwk:Hours>15.5</pwk:Hours>

</pwk:Worker>

</pwk:Workers>

</pwk:Project>

…

</pwk:Projects>

<?xml version=“1.0” encoding=“UTF-8”?>

<xsd:schema xmlns:xsd= http://www.w3.org/2001/XMLSchema xmlns:pwk=http://www.example.org/ProjectWorkers

elementFormDefault="qualified">

<xsd:element name=“Projects”>

<xsd:complexType>

<xsd:sequence>

<xsd:element name=“Project” minOccurs=“1”

maxOccurs=“unbounded”>

<xsd:complexType>

<xsd:sequence>

<xsd:element name=“Name” type=“xsd:string”/>

<xsd:element name=“Location” type=“xsd:string”/>

<xsd:element name=“Dept_no”

type=“xsd:positiveInteger” minOccurs=“0 “ maxOccurs=“1”/>

<xsd:element name=“Workers” minOccurs=“0”

maxOccurs=“unbounded”>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

<xsd:attribute name="number"

type="xsd:positiveInteger"/>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

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XML Schema

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<xsd:schema xmlns:xsd= http://www.w3.org/2001/XMLSchema xmlns:pwk=

http://www.example.org/ProjectWorkers elementFormDefault="qualified">

<xsd:complexType>

<xsd:sequence>

<xsd:element name=“Project” minOccurs=“1” maxOccurs=“unbounded”>

<xsd:complexType>

<xsd:sequence>

<xsd:element name=“Dept_no” type=“xsd:positiveInteger” minOccurs=“0 “ maxOccurs=“1”/>

<xsd:element name=“Workers” minOccurs=“0” maxOccurs=“unbounded”>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

<xsd:attribute name="number" type="xsd:positiveInteger"/>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Declare the xsd:schema element

This says we’re creating an XML Schema Our namespace will be “pwk”

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Elements are declared with the “element” tag

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Complex elements use the complexType tag to hold their component parts

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

The child elements will form a sequence

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

There will be at least one Project element and no maximum number of Project elements

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Project is another complexType holding a sequence of elements

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Name, Location, Dept_no are all simple elements

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Workers is another complex element

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XML Schema

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<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

<xsd:sequence>

<xsd:complexType>

…

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:sequence>

</xsd:complexType>

</xsd:element>

</xsd:schema>

Project has a required attribute “number”

with a positive Integer type

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Storing and Extracting XML Documents from Databases



Most common approaches

◦ Using a DBMS to store the documents as text

 Can be used if DBMS has a special module for document processing

◦ Using a DBMS to store document contents as data elements

 Require mapping algorithms to design a database schema that is compatible with XML document structure

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Storing and Extracting XML

Documents from Databases (cont’d.)

◦ Designing a specialized system for storing native XML data

 Called Native XML DBMSs

◦ Creating or publishing customized XML documents from preexisting relational databases

 Use a separate middleware software layer to handle conversions

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XML Languages



Several different query languages proposed for XML

◦ Languages used to select (and manipulate) the nodes in an XML document



Common tools for querying data in XML documents

◦ XPath

◦ XQuery

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XML Languages



Two query language standards

◦ XPath

 Specify path expressions to identify certain nodes (elements) or attributes within an XML document that match specific patterns

◦ XQuery

 Uses XPath expressions but has additional constructs

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XPath



XPath is a simple query language used to select parts of an XML

document

◦ Other more complex languages

(XQuery, XSLT) use the XPath syntax



XPath queries are written as

paths through an XML document

◦ Nodes are separated by ‘/’

characters

◦ The result returned is whatever is at

the end of the XPath expression

³⁶

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XPath: Specifying Path Expressions in XML



XPath expression

◦ Returns a sequence of items that

satisfy a certain pattern as specified by the expression

◦ Either values (from leaf nodes) or elements or attributes

◦ Qualifier conditions

 Further restrict nodes that satisfy pattern



Separators used when specifying a path:

◦ Single slash (/) and double slash (//)

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Xpath:



/company

◦ Returns the company root node and all its descendant nodes

 the whole XML document.

◦ Customary to include the file

 E.g.

 doc(www.company.com/info.XML)/company

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XPath: Specifying Path

Expressions in XML (cont’d.)



Main restriction of XPath path expressions

◦ Path that specifies the pattern also specifies the items to be retrieved

◦ Difficult to specify certain conditions on the pattern while separately

specifying which result items should

be retrieved

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XPath: Specifying Path

Expressions in XML (cont’d.)



/ - child of the current node



// - descendent or self at any level of the current node



@ - attribute of the current node



* - Wildcard symbol

◦ Stands for any element

◦ Example: /company/*

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Xpath (cont’d)



/company/department

◦ Returns all department nodes and their descendant subtrees.

◦ Nodes in an XML document are ordered, so the XPath result will be in the same order as the document tree.

 //employee [employeeSalary gt 70000]

/employeeName

◦ [qualifier condition]

◦ // if we don’t know the full path, but do know the name of some tags

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XPath: Specifying Path

Expressions in XML (cont’d.)



Axes

◦ Move in multiple directions from current node in path expression

◦ Include self, child, descendent,

attribute, parent, ancestor, previous

sibling, and next sibling

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XQuery



XQuery is an extension of XPath

◦ Uses the same data model as XPath

◦ Provides a language for more complex and general queries

◦ Similar to SQL’s relationship to relational databases



XQuery uses FLWR expressions

◦ Acronym for “For, Let, Where, Return”

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XQuery Features



Support for joins

for $x in <element1>, $y in <element2>

where $x/element = $y/element



Support for aggregate functions

◦ Sum, count, min, max, avg



Support for conditional branching

◦ If – then branching



And a lot more

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XQuery: Specifying Queries in XML



XQuery FLWR expression

◦ Four main clauses of XQuery

◦ Form:

FOR <variable bindings to individual nodes (elements)>

LET <variable bindings to

collections of nodes (elements)>

WHERE <qualifier conditions>

RETURN <query result specification>

◦ Zero or more instances of FOR and

LET clauses

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Xquery Example

LET $d :=

doc(www.company.com/info.xml) FOR $x IN

$d/company/project[projectNum ber = 5]/projectWorker, $y IN

$d/company/employee

WHERE $x/hours gt 20.0 AND

$y.ssn = $x.ssn RETURN <res>

$y/employeeName/firstName,

$y/employeeName/lastName,

$x/hours </res>

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Variables have $ prefix LET assigns a variable for the rest of the query

FOR assigns a variable to range over items in a

sequence

WHERE specifies any conditions

RETURN specifies the elements to be retrieved.

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Summary



Three main types of data:

structured, semi-structured, and unstructured



XML standard

◦ Tree-structured (hierarchical) data model

◦ XML documents and the languages for specifying the structure of these documents



CSE 3241: XML Extensible Markup Language (Ch. 12)

CSE 3241: XML

Extensible Markup Language

(Ch. 12)

Topics

Structured, Semistructured, and Unstructured Data

XML Hierarchical (Tree) Data Model

XML Documents, DTD

XML Schema

Storing and Extracting XML Documents from Databases

XML Languages

Structured, Semistructured, and Unstructured Data

Structured data

◦ Represented in a strict format

◦ Example: information stored in databases

Semistructured data

◦ Has a certain structure

◦ Not all information collected will have identical structure

Unstructured data

◦ Limited indication of the of data

document that contains information

embedded within it

What is XML?

XML provides a framework to define a structure for data

◦ An XML document is a collection of related data items

◦ Document is “marked up” with tags known as elements

XML Hierarchical (Tree) Data Model

Elements and attributes

Complex elements

Simple elements

XML tag names

XML Hierarchical (Tree) Data Model (cont’d.)

XML attributes

◦ Describe properties and

characteristics of the elements (tags) within which they appear

◦ May reference another element in another part of the XML document

The XML Data Model

Attributes vs. Elements

XML Document Types – Data Centric XML

Data-centric XML

◦ Highly structured

◦ Many small data items

◦ Often used for data exchange purposes

◦ Also used to create web pages dynamically from databases

◦ Generally follow a schema document that determines their structure

XML Document Types – Document-Centric XML

Few structural elements

Large amounts of text

May have a schema document, but not required

More XML Document Types

Hybrid XML

◦ Some parts are highly structured

◦ Some parts mostly blocks of text and/or unstructured

◦ May or may not have a predefined schema

Schemaless XML documents

◦ Semi-structured documents without a predefined schema

◦ Denoted by the attribute

‘standalone=“yes”’ in the XML declaration on the top line

Valid XML

An XML document is considered valid if:

◦ It is well-formed* , and …

◦ It follows a particular schema in a standard definition language

*Well-formed XML

An XML document is well-formed when it follows certain conditions:

DTD – Document Type Definition

Original method of specifying a schema definition

◦ Still in widespread use

A very simple schema definition language

◦ Each possible element in the document is defined

XML Documents, DTD, and XML Schema (cont’d.)

Notation for specifying elements

XML DTD

◦ Special syntax

◦ All DTD elements always forced to follow the specified ordering of the document

A sample XML document and DTD

XML Document Types - Schemas

Semi-structured data

Defines

Communicate semantic information about an XML document

XML Schema