Module 5: Querying XML

Explain that students can either compile the query into an XPathExpression object and then use the Select method of the XPathNavigator object, or they can execute a string XPath expressi

Contents

Overview 1 Lesson: Introduction to Querying XML

Module 5:

Querying XML

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Instructor Notes

This module explains the process of querying an Extensible Markup Language (XML) document by using the XML Path Language (XPath) It explains how to write code to load an XML document and how to create a navigation cursor by

using an XPathNavigator object It also explains how to create your query by using an XPathDocument object and how to examine the result types In

addition, this module describes how to compile your query and evaluate your results

After completing this module, students will be able to:

Describe the business reasons for querying Extensible Markup Language (XML) documents by using the XML Path Language (XPath)

Cache XML data for querying by using an XPathDocument object

Create an XPathNavigator object to navigate through an XML document

Compile and evaluate the result of an XPathExpression object

Iterate through an XmlNodeList object returned by an XPath expression

To teach this module, you need the following materials:

Microsoft® PowerPoint® file 2663A_05.ppt

2663A_05_code.htm

Module 5, “Querying XML”

Instructor-led demonstration file

To prepare for this module:

Read all of the materials for this module

Complete the lab

Practice the instructor-led demonstration and code examination

The demonstrations in this module consist of a demonstration and code walkthrough of querying an XML document To prepare for these demonstrations, perform the demonstration steps as they are written and prepare to explain how the code works

In this module some of the PowerPoint slides provide hyperlinks that open a code samples page in the Web browser The code samples page provides a way

to show and discuss code samples when there is not enough space for the code

on the PowerPoint slide It also allows students to copy code samples directly from the browser window and paste them into a development environment All

of the linked code samples for this module are in a single htm file

To open a code sample, click the appropriate hyperlink on the slide To navigate between code samples in a particular language, use the table of contents

provided at the top of the code page Each hyperlink opens a separate instance

the Web browser, so it is a good practice to click Back in Internet Explorer

after you view a code sample This will close the browser window and return you to the PowerPoint presentation

How to Teach This Module

This section contains information that will help you to teach this module

Lesson: Introduction to Querying XML Using XPath

This section describes the instructional methods for teaching each topic in this lesson

This lesson introduces XPath and the reasons for using it To understand the rest of the module, the students must understand how XPath expressions are a navigational language for XML documents Stress that XPath is an XML language that provides access to nodes and their contents, wherever the nodes are located in an XML document

The role of the World Wide Web Consortium (W3C) in the development of XPath in conjunction with the development of XQuery illustrates the dynamic nature of XML technologies and languages Stress that this evolution is ongoing and requires continual adjustments and refinements by both vendors and developers However, stress that the development is not random

This topic introduces students to the steps in the query process Emphasize that these steps are general For example, students might compile the query or they might not Explain that this information is covered in the third lesson of this module

In this demonstration, you will load a product catalog document and demonstrate multiple queries These queries have different return types, including node sets, numbers, strings, and Boolean values

Lesson: Creating and Navigating a Document Cache

This section describes the instructional methods for teaching each topic in this lesson

This topic explains why to use an XPathDocument object to cache data Emphasize that using the XPathDocument object provides a way for students

to query data in a read-only manner This topic also explains how to preserve

all white space in the source XML document Explain how XmlSpace.Preserve and XmlSpace.Default function

To navigate through an XML source document, students must create a cursor

by creating an XPathNavigator object In this topic, students will learn how to use the CreateNavigator() method to create an XPathNavigator object

This topic defines five methods that students can use to position the navigator

by traversing the XML document

Why Query XML Using

Lesson: Executing Your Query

This section describes the instructional methods for teaching each topic in this lesson

In this topic, students will learn how to create and compile a query by using the

Compile() method Emphasize that one advantage of compiling a query is that

doing so allows you to determine the data type of the information returned by the query

This topic explains how to use the Evaluate() method to evaluate number return types Emphasize that the Evaluate() method returns a typed result

This topic explains how to use the Select() methods to evaluate node return

types Explain that students can either compile the query into an

XPathExpression object and then use the Select() method of the XPathNavigator object, or they can execute a string XPath expression directly

by using one of the Select() methods of the XPathNavigator object

Students might have noticed in the Select() method example in the previous topic that the resulting nodes are stored in an XPathNodeIterator This topic explains how to use an XPathNodeIterator to iterate through a node set Emphasize that students must call the MoveNext() method to move the XPathNodeIterator to the first node in the selected set

Explain the code underlying the application that you demonstrated at the beginning of this module

of knowing the right answers

1 If the students are not sure what method to call to move the

XPathNodeIterator to the first node in the selected set, review the third

4 This question explores the distinctions between the Evaluate() and Select()

methods

How to Create and

Compile a Query

How to Execute XPath

Expressions That Return

Numbers

How to Execute XPath

Expressions That Return

Node Sets

How to Iterate Through a

Node Set

Code Examination

Lab 5.1: Querying XML Documents Using XPath

Before beginning this lab, students should have completed the module and answered the review questions Students will need to be able to perform most of the tasks that they learned in the lessons

Customization Information

This section identifies the lab setup requirements for a module and the configuration changes that occur on student computers during the labs This information is provided to assist you in replicating or customizing

Microsoft Official Curriculum (MOC) courseware

Overview

Introduction to Querying XML Using XPath

Creating and Navigating a Document Cache

Executing Your Query

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This module explains the process of querying an Extensible Markup Language (XML) document by using the XML Path Language (XPath) It explains how to write code to load an XML document and how to create a navigation cursor by

using an XPathNavigator object It also explains how to create your query by using an XPathDocument object and how to examine the result types In

addition, this module describes how to compile your query and evaluate your results Because an XPath expression can return a set of nodes, you will learn

how to iterate through a set of nodes by using an XPathNodeIterator object

After completing this module, you will be able to:

Describe the business reasons for querying XML documents by using XPath

Cache XML data for querying by using an XPathDocument object

Create an XPathNavigator object to navigate through an XML document

Compile and evaluate the result of an XPathExpression object

Iterate through an XmlNodeList object returned by an XPath expression Introduction

Objectives

Lesson: Introduction to Querying XML Using XPath

What Is the Query Process?

Demonstration: Querying XML Using XPath

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In this lesson, you will gain an understanding of XPath and how you can use XPath to locate parts of XML documents and navigate through a document that

is represented as a tree of nodes

After completing this lesson, you will be able to:

Describe the business reasons for querying XML documents by using XPath

Describe the steps in the querying process

Introduction

Lesson objectives

Why Query XML Using XPath?

You can use XPath to:

Search

Filter

Summarize

XPath : XML :: Transact-SQL : SQL Server

Transact-SQL provides a standard language for manipulating relational database objects

XPath provides a standard language for navigating an XML document as a hierarchical tree of nodes

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XPath is a programming language developed by the World Wide Web Consortium (W3C) specifically to address the nodes of an XML source XPath

is also well suited to querying an XML source You can use XPath to extract from an XML source whatever pattern of content or XML structure you require Some of the functionality of XPath is similar to the functionality of the

Document Object Model (DOM) For example, by using the DOM, you can locate a certain piece of XML data by node type However, the use of XPath to specify location information in an XML document is far more pervasive than the use of the DOM What then is XPath, and why is it the technology of choice for querying XML?

XPath was developed in part to correct some of the limitations inherent in querying XML by using the DOM XPath code that addresses and navigates through nodes is much simpler than similar code written for the DOM

XPath allows you to navigate through the nodes of an XML source You can also combine XPath navigation with other programming techniques For example, you can use XPath to locate parts of an XML document to generate results for a query with XQuery and XML Views in Microsoft®

SQL Server™ 2000

Introduction

Why use XPath?

The following table illustrates how you can use XPath

Example Action Microsoft NET Framework objects used

Get all the <ProductName>

elements in a document with a price attribute value > $12.95

Search An XPathDocument object is

optimized for query Use an XPath expression to locate nodes

Change price information in

an XML product catalog

Locate information

to modify

An XmlDocument object provides an

implementation of the W3C DOM Use an XPath expression to locate nodes to be modified

Only find what matches a specified search criterion

Filter You can apply an Extensible

Stylesheet Language for Transformation (XSLT) style sheet to

an XPathDocument object The

XSLT style sheet often contains an XPath expression

Get a count of the number of parts that are out of stock

Summarize You can query an XPathDocument

object to provide a count or sum of information in the document, by using

an XPath expression

Examples of using

XPath

What Is the Query Process?

MoveToRoot( ) MoveToNext( ) MoveToPrevious( ) MoveToFirstChild( )

XPathDocument CreateNavigator XPathNavigator

Evaluate( ) Select( )

Select( )

Compile( )

Boolean String Double

NodeSet

XPathNodeIterator MoveNext( ) For Each XPathExpression

5 Check the ReturnType

1 Load XPathDocument

2 Create XPathNavigator 4 Compile XPathExpression

3 Create the Query

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Querying by using XPath provides a way for your application to navigate from one node to the next, and to perform various operations along the way

To query an XML document, you generally perform the following steps:

1 Write code to load an XML document into an appropriate cache

You can use an XmlDocument object, XPathDocument object, DataSet,

or XmlDataDocument, depending on how you want to access the

information

2 Create a navigator to navigate through the XML document

You can use an XPathNavigator object to efficiently select nodes in an

XML document

3 Create the query

You generally create an XPath expression to select nodes in the cache You can execute a query directly, or you can compile it if you expect to run the query multiple times

4 Optionally, compile the query and determine the result type

XPath queries can return nodes, strings, or other types of values You must either know the types of results that you expect the query to produce, or determine the return type programmatically

If you compile the query, you can determine the type of information that will be returned before you execute the query In addition, compiling the query will result in faster execution

5 Evaluate the results

Depending on the type of information returned, you can process results by iterating through a set of nodes, or simply process the values returned

Introduction

Querying an XML

document

Demonstration: Querying XML Using XPath

Load document - XPathDocument Create navigator - XPathNavigator Create query - XPathExpression

Optionally, compile query and examine result type

Optionally, compile query and examine result type

Evaluate results

Evaluate results

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In this demonstration, you will see how you can query an XML document containing product catalog information The application:

Loads an XML document into a cache optimized for query

Creates a navigator to navigate nodes in the document

Compiles an XPath query and determines the return type

Evaluates the query and generates results

Processes and displays the results

You should also use this demonstration to review simple XPath location paths and functions

To load the product catalog document

1 In Microsoft Internet Explorer, open ProductCatalogData.xml and examine the structure of the document Notice that each category element contains category information and a product element containing all products in that category

2 In Microsoft Visual Studio® NET, open the project file for the Querying demonstration Open the C# or Microsoft Visual Basic® project file as appropriate

3 Run the application

4 Load the data file ProductCatalogData.xml by using the application The

application loads data from the file into an XPathDocument object

Introduction

Demonstration

To compile and evaluate queries that return node sets

• For each XPath expression in the following table, click Evaluate XPath, examine the return type, and then click Execute XPath and explain the

results

any level

/node()/node() The element or text nodes that are

grandchildren of the current node /node()/node()/node() The element or text nodes that are

great-grandchildren of the current node//Product/ProductName All product name elements that are

descendants of a product element //AllCategories/Category/CategoryName All category names

//Category[CategoryID = 2]/

Products/Product/ProductName

All product names in category 2

To compile and evaluate queries that return numbers

• For each XPath expression in the following table, click Evaluate XPath, examine the return type, and then click Execute XPath and explain the

results

count(node()/node()) Number of children of the root element

count(//Category[CategoryID = 2]/

Products/Product)

Number of products in category 2

To compile and evaluate queries that return strings

• The following XPath expression concatenates the first three grandchildren

nodes of the current node Click Evaluate XPath, examine the return type, and then click Execute XPath and explain the results

concat(node()/node()/node()[1],

" ",node()/node()/node()[2], " ",node()/node()/node()[3])

To compile and evaluate queries that return Boolean values

1 For each XPath expression in the following table, click Evaluate Xpath, examine the return type, and then click Execute XPath and explain the

results

starts-with(//Product/ProductName[1], "C") Does the first product name begin

with the letter “C”? (yes) starts-with(//Product/ProductName[1], "D") Does the first product name begin

with the letter “D”? (no)

2 Close the application

Lesson: Creating and Navigating a Document Cache

What Is an XPathDocument Object?

How to Create a Navigator

How to Navigate Nodes in an XPath Document

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In this lesson, you will gain an understanding of how to prepare to query an XML document To query an XML document, you must load the XML

document and create a navigator by creating an XPathNavigator object

After completing this lesson, you will be able to:

Load an XML document into an XPathDocument object

Create a navigator by creating an XPathNavigator object

Navigate an XPathDocument by using XPathNavigator methods

Introduction

Lesson objectives

What Is an XPathDocument Object?

You can create an XPathDocument from:

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Before you can query XML data, you must first load the data into a cache The Microsoft NET Framework contains several objects that can cache XML data

These include the XmlDocument, XPathDocument, and XmlDataDocument

objects The choice of which cache to use depends on what you need to do with the data

If you expect to query data in a read-only manner from an XML source, you

should use an XPathDocument object to cache the data The XPathDocument

object provides a read-only, in-memory cache for XML data that is optimized for querying with XPath and for applying XSLT style sheets You can also use

an XPathDocument object if you want to navigate through the individual

nodes in the document

You can also query, navigate, and apply XSLT transformations to data in an

XmlDocument object However, using the XmlDocument rather than the XPathDocument object is not as efficient for querying and applying style sheets XmlDocument is the NET Framework implementation of the DOM

and is intended for modifying the data

You can create a new XPathDocument object and populate it with data by

using the object’s constructor This constructor is overloaded to accept data

from a stream, file, TextReader, or XmlReader

The following example illustrates how to create a new XPathDocument object:

the XPathDocument object, when the data source is a file or an XmlReader

The optional parameter XmlSpace can have the following values

Value Description

XmlSpace.Default Preserve only the significant white space

XmlSpace.Preserve Preserve all white space

XmlSpace.None Do not preserve white space

The following example illustrates how to preserve all white space in the source XML document:

// C#

//Create an XmlTextReader XmlTextReader CatalogReader;

//Code to populate the reader from a file…

How to Create a Navigator

Create an XPathNavigator object

Use the CreateNavigator method

C# Example Visual Basic Example

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To create a query, you must create a navigation cursor to navigate through the XML document you want to query You create a navigator by using the

CreateNavigator() method to create an XPathNavigator object

XPathNavigator provides read-only, random access to the data in the XML

document you loaded

The XPathNavigator object is based on the XPath data model and provides the

methods required to implement XPath queries over any data store You can also

use XPathNaivgator methods to traverse the nodes in the cache

To create an XPathNavigator object, use the CreateNavigator() method of the XPathDocument class The XPathNavigator class implements the

IXPathNavigable interface

Introduction

How to create an

XPathNavigator

The following example creates an XPathNavigator object:

'Create and populate an XPathDocument Dim doc As XPathDocument = New XPathDocument(fileName) 'Create a navigator on the document

Dim nav As XPathNavigator = doc.CreateNavigator()

The XPathNavigator object has methods for selecting a set of nodes based on

criteria in an XPath expression You can also use methods to navigate in an

XPathDocument object in a way similar to how you traverse a DOM tree You will see how to use the Select() methods with an XPath expression in the

next lesson, Executing Your Query

You will see how to traverse nodes by using navigational methods in the next topic

Example

Methods for returning

nodes

How to Navigate Nodes in an XPathDocument

Moves the navigator to the first child of

Moves the navigator to the next sibling

of the current node

C# Example Visual Basic Example

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After you create an XPathNavigator over an XPathDocument, you can

traverse the nodes in the document or access individual nodes in a access manner You position the navigator on individual nodes by using various

random-methods of the XPathNavigator class

When you use an XPathNavigator, there is a concept of the current node You can set the current node in two primary ways: by using an XPathExpression object as part of a Select() method, or by using use navigational methods You will use the Select() methods in the next lesson You can use the following

navigational methods to position the navigator by traversing the XML

Method Action MoveToRoot() Moves the navigator to the root of the document

MoveToNext() Moves the navigator to the next sibling of the current node

MoveToPrevious() Moves the navigator to the previous sibling of the current

node

MoveToFirstChild() Moves the navigator to the first child of a node Only valid on

nodes that have children

MoveToParent() Moves the navigator to the parent of the current node Not

valid for the root node, because the root node has no parent

Introduction

How to traverse

document nodes by

using an XPathNavigator

The following example uses the navigational methods of the XPathDocument

object to traverse the nodes in the document and print out properties and node values:

// C#

//Examine the current node in an XPathDocument //The XPathNavigator is positioned at the current node public static void RecursiveWalk(XPathNavigator nav) {

switch (nav.NodeType){

//The node is an element node //Print out information case XPathNodeType.Element:

if (nav.Prefix==String.Empty) MessageBox.Show(nav.LocalName);

else MessageBox.Show (nav.Prefix, nav.LocalName);

if ( nav.MoveToFirstChild() ) {

//If children exist, process the child recursively

do { RecursiveWalk(nav);

} //get the next sibling while ( nav.MoveToNext() );

//get the parent node nav.MoveToParent();

if (nav.NodeType == XPathNodeType.Element) MessageBox.Show (nav.Name);

} }

Code continued on the following page

Example of node

navigation

'Visual Basic Public Shared Sub RecursiveWalk(nav as XPathNavigator) Select Case nav.NodeType

Case XPathNodeType.Element

If (nav.Prefix=String.Empty) Then MessageBox.Show(nav.LocalName) Else

MessageBox.Show(nav.Prefix,nav.LocalName) MessageBox.Show(nav.NamespaceURI)

End If Case XPathNodeType.Text MessageBox.Show(nav.Value) End Select

If (nav.MoveToFirstChild()) Then

Do RecursiveWalk(nav) Loop While (nav.MoveToNext()) nav.MoveToParent()

If (nav.NodeType = XPathNodeType.Element) Then MessageBox.Show (nav.Name)

End If End If End Sub

Lesson: Executing Your Query

How to Create and Compile a Query

How to Execute XPath Expressions That Return Numbers

How to Execute XPath Expressions That Return Node Sets

How to Iterate Through a Node Set

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After you load the XML document you want to query and create your navigator, you can define your search criteria, examine the result type, and evaluate your results In this lesson, you will see how to create and compile your query, how to evaluate your results, and how to iterate through those results if your return type is a node set

After completing this lesson, you will be able to:

Create a query by creating an XPathExpression object

Execute XPath expressions that return numbers

Execute XPath expressions that return node sets

Create an XPathNodeIterator object to iterate through node sets

Introduction

Lesson objectives

How to Create and Compile a Query

To create a query, create an XPathExpression object

To create an XPathExpression object, use the Compile method of the XPathNavigator object

To determine the return type, examine the ReturnType property

Visual Basic Examples C# Examples

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To create a query that you intend to execute more than once, you should create

an XPathExpression object The XPathExpression class encapsulates a compiled XPath expression You create an XPathExpression object when you

compile a query You can create and execute queries without compiling them,

as explained later in this module

There are two primary reasons for compiling a query rather than executing it

directly by using an XPathNavigator

If you compile a query before executing it, you can determine the data type of the information to be returned XPath queries can generate single nodes, node sets, strings, Booleans, or numbers It is important to know what information will be returned so that you can capture and process it properly

Another advantage of compiling the XPathExpression is that the query will

execute faster than the non-compiled version This is useful if you expect to execute the query multiple times in an application

Introduction

Why compile a query?

To compile a query, you create an XPathExpression object and then use the Compile() method of the XPathNavigator object with an XPath query You set the XPathExpression object to the result

'Compile the expression by using the Compile() 'method of a populated XPathNavigator

xpe = nav.Compile("//book/author")

After you compile an XPathExpression, you can determine the data type of the result by using the ReturnType property The following table shows the possible values of the ReturnType and a description of each value

Description and enumerated value Value

String 0 Boolean 1 Number 2

Error 4 Any 5

How to compile a query

Determining the return

type