Introduction to XPath

Kristian Torp

Department of Computer ScienceAalborg University

people.cs.aau.dk/˜torptorp@cs.aau.dk

November 3, 2015

daisy.aau.dk

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 1 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 2 / 59

Learning Goals and Focus

Learning GoalsUnderstand the XPath data model

Know the basic tree terminology

Good at querying XML documents using XPathKnow the abbreviations used in XPath

Very handy to know in practiceCompact and quite readable!

Database FocusAll XML technologies are presented from a database perspective also

called a data focus (i.e., not a document focus)!

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 3 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 4 / 59

Introduction

ExampleFind all courses: /coursecatalog/course

Find the semesters: //semester/text()

OverviewA language for

finding/addressing information in XML documentsnavigating through elements and attributes in an XML document

Used in many XML technologies, e.g., XQuery and XPointerA part of the XSLT recommendation

Microsoft/Visual Studio makes heavy usage of XSLT

The data model is an abstract and logical structure of an XMLdocument

Called a node tree

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 5 / 59

The Node Tree

TerminologyDocument node: The entire XML document

Also called the document root or the root nodeElement node: An XML element

A special one is the document element or root element

Text node: The text strings in an element nodeAttribute node: An attribute

Example (A Node Tree)/

coursecatalog

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 6 / 59

Example: Find the CoursesExample (Document)

/

coursecatalog

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

Query/coursecatalog/course

Result

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 7 / 59

Example: Find the Semesters

Example (Document)

/

coursecatalog

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

Query//semester/text()

Result

3 7

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 8 / 59

Major Components

ComponentsNodes

XML document treated as a tree of nodesExamples: Elements, attributes, and comments

Path expressionsSelect a set of nodes in an XML documentExamples: /, /coursecatalog/course

Standard functionsApproximate 100 built-in functionsExamples: concat(’a’, ’b’), round(1.5)

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 9 / 59

Quiz

Example (Document)

/

coursecatalog

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

QuestionsWho is the parent of the document element?

How many document elements are there in an XML document?

How many elements can there be in an XML document?

Are elements and attributes the same node type?Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 10 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 11 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

TreeLike any other tree in CS

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Root

QuizAre there other roots?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Leafs

QuizAre there more leafs?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Children of 1

QuizWho are the children of 3?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Siblings of 9

QuizWho are the siblings of 3?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Ancestors of 6

QuizWho are the ancestors of 9?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Parent of 8

QuizWho are the parents of 4?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

Tree Terminology

Example (A Node Tree)

1

2

3 4

5

6

7

8

9 A B

Descendants of 1

QuizWho are the descendants of 5?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 12 / 59

QuizExample (Another Node Tree)

1

2

3 4

5 6 7

8

9

A

B

C

D E F

G

H I

J

QuestionsParent of E?

Children of 2?

Descendants of 2?Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 13 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 14 / 59

Location Path and Location Step I

Definition (Location Path)A location path evaluates to a sequence of nodes

Example (Location Path)/child::coursecatalog/child::course[name=’OOP’or name=’DB’][@id<10]

Definition (Location Step)A location path consists of a number of location steps.

Example (Location Steps)child::coursecatalog

child::course[name=’OOP’or name=’DB’][@id<10]

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 15 / 59

Location Path and Location Step II

DefinitionA location step consists of an axis, a node test, and a set of predicates

Example (One)child::coursecatalog

Axis: childNode test: coursecatalogPredicates: empty

Example (Two)child::course[name=’OOP’or name=’DB’][@id<10]

Axis: childNode test: coursePredicates: [name=’OOP’or name=’DB’][@id<10]

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 16 / 59

Abbreviations

Most UsedAbbreviation Meaning

. self::node()

.. parent::node()

//coursecatalog /descendant-or-self::coursecatalog

course child::course

Example (Abbreviations in Action)

Abbreviation Meaning

//name /descendant-or-self::name

//name/.. /descendant-or-self::name/parent::node()

/coursecatalog/course /child::coursecatalog/child::course

NoteAbbreviations makes the expression more readableSometimes abbreviations can make it hard to guess the result

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 17 / 59

Evaluation of Location Path I

Example XML Document

/

coursecatalog

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

Evalute the Location Path/child::coursecatalog/child::course[name=’OOP’or name=’DB’][@id<10]/name

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 18 / 59

Evaluation of Location Path II

The Steps in the Evaluation1 Starts with / therefore the context node is set to root node2 Evaluate the location step child::coursecatalog3 Result is the coursecatalog root element node4 Set context to root element node5 Evaluate the location stepchild::course[name=’OOP’or name=’DB’][@id<10]

6 The result is the two course element nodes7 Set context to the OOP course element node8 Evaluate the location step child::name9 Results in the name element node which is the first part of the result

10 Set context to the DB course element node11 Evaluate the location step child::name12 Results in the name element node which is the last part of the resultKristian Torp (Aalborg University) Introduction to XPath November 3, 2015 19 / 59

Context

Definition (Context)A context node (a node in the node tree)

A context size and context position

A set of variable bindings

A function library

A set of name space declaration

Definition (Context Size)The context size is the lenght of the sequence of nodes return by theprevious location step

Definition (Context Position)The context position is the current node in the sequence being evaluated

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 20 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 21 / 59

Compact Notation for Node TreeExample (The Node Tree)

/

coursecatalog

course

id=4 name

OOP

semester

3

desc

snip

course

id=2 name

DB

semester

7

desc

snip

Example (The Equivalent Compact Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 22 / 59

Example: Find the Courses

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/course

Result

course:OOP

id=4 name:OOP sem:3 dsc

course:DB

id=2 name:DB sem:3 dsc

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 23 / 59

Example: Find Elements That Do Not Exist

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/name

ResultEmpty no name element below coursecatalog!

Note that it is not an error!

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 24 / 59

Example: Find the Course Names

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog//name

Result

name:OOP name:DB

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 25 / 59

Examples: Find the OOP Course

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/course[name="OOP"]

Result

course:OOP

id=4 name:OOP sem:3 dsc

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 26 / 59

Example: Find a Course Based on ID

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/course[@id="2"]

Result

course:DB

id=2 name:DB sem:7 dsc

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 27 / 59

Example: Filter on an Attribute

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/course[@id="2"]/name

Result

name:DB

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 28 / 59

Example: Get the Name of a Course as a String

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/course[@id="2"]/name/text()

ResultThe string DB

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 29 / 59

Example: Use Parent Axis

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query//course[@id="2"]/parent::node()

ResultThe document node, i.e., the entire tree

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 30 / 59

Example: Use Child Axis

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/child::node()

Result

course:OOP

id=4 name:OOP sem:3 dsc

course:DB

id=2 name:DB sem:7 dsc

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 31 / 59

Example: Use Descendant Axis

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Query/coursecatalog/descendant::node()

Result8 element nodes

6 text nodes

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 32 / 59

Example: Use Functions

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queryconcat("hello, ", "world!")

ResultThe string ’hello, world!’

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 33 / 59

Example: Functions and XPath Expressions

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queryconcat("hello ", /coursecatalog/course[@id="2"]/name/text())

ResultThe string ’hello DB’

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 34 / 59

Most used Path Expressions

Often Used Expressions

Path Expression Description

/ select from the root node//NodeName select NodeName element nodes. select the current node.. select parent of the current node/NodeName[@id>7] select based on attribute node/NodeName[Node2=’H’] select based on element node/NodeName/text() select the text node value/NodeName/attribute() select the attribute nodes/NodeName[1] select the first NodeName element node/NodeName[last()] select the last NodeName element node

NoteAlmost like Linux/Unix directory navigation

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 35 / 59

Quiz

Example (Node Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Questions/coursecatalog/course/name returns?

/coursecatalog/teacher returns?

/coursecatalog is the same as /?

/coursecatalog/course/../course/../course returns?

/coursecatalog/course[@id<11]/name/text() returns?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 36 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 37 / 59

Node Numbering

Example (Node Tree)

1

2

3 4

5

6

7

8 9

10 11

12 13 14

NoteDepth-first numbering of nodes

Used for relative access to other nodes

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 38 / 59

Forward and Backward Axes

Definition (Axis)An axis is a sequence of nodes located relative to the context node.

Definition (Forward Axis)A forward axis can only return the context node or nodes after in thedocument order.

Definition (Backward Axis)An backward axis can only return the context node or nodes that arebefore in the document order.

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 39 / 59

The Axes

Axis Name Direction Description

attribute forward All my attributesself forward My selfchild forward All my childrendescendant forward All my children, grand children, etc.parent backward My unique parentancestor backward My parent, grand parent, etc.following forward All after me that are not ancestorspreceding backward All before me that are not ancestorsfollowing-sibling forward My “younger” siblingspreceding-sibling backward My “elder” siblingsdescendant-or-self forward My self and all my descendantsancestor-or-self backward My self or my ancestors

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 40 / 59

Child

FindsImmediately descendants to current node.

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

cur

1 2 3

QuizWhich direction of the child axis (and why)?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 41 / 59

Child ExamplesExample (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/child::node()

Result: the two course nodes

/coursecatalog/course/child::node()

Result: six element nodes

/coursecatalog/course/attribute()

Result: two attribute nodes

/coursecatalog/course/semester/child::node()

Result: two text nodesKristian Torp (Aalborg University) Introduction to XPath November 3, 2015 42 / 59

ParentFinds

The one node immediately above

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

1

cur

QuizWhich direction of the parent axis (and why)?

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 43 / 59

Parent ExamplesExample (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/course[@id=’2’]/name/parent::node()

Result: the course element node with id = 2

/coursecatalog/course/name/parent::node()

Result: the two course element nodes

/coursecatalog/parent::node()

Result: the document root

/parent::node()

Result: emptyKristian Torp (Aalborg University) Introduction to XPath November 3, 2015 44 / 59

Descendent

FindsChildren all the way down the tree

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

cur

1

2 3

4 5

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 45 / 59

Descendant ExamplesExample (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/descendant::node()

Result: 8 element nodes + 6 text nodes

/coursecatalog/course[name="OOP"]/descendant::node()

Result: 3 element nodes + 3 text nodes

/coursecatalog/course[name="OOP"]/descendant::node()/attribute()

Result: 2 attribute nodes

/coursecatalog/course/name/descendant::node()

Result: two text nodesKristian Torp (Aalborg University) Introduction to XPath November 3, 2015 46 / 59

Ancestor

FindsParents all the way up the tree

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

4

3

2

1

cur

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 47 / 59

Ancestor ExamplesExample (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/course[name="DB"]/desc/ancestor::node()[2]

Result: course-catalog element node

/coursecatalog/course/name/ancestor::node()

Result: document node + coursecatalog node + 2 course nodes

/coursecatalog/ancestor::node()

Result: document root

/ancestor::node()

Result: empty

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 48 / 59

Following

FindsAll nodes that follows excluding descendants

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

cur 1 2

3 4 5

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 49 / 59

Following Examples

Example (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/course[@id="4"]/following::node()

Result: 4 element nodes + 3 text nodes

/coursecatalog/course[@id="2"]/following::node()

Result: empty

/coursecatalog/course[@id="4"]/name/text()/following::node()

Result: 6 element nodes and 5 text nodes

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 50 / 59

Preceding

FindsAll preceding nodes excluding ancestors

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

3

2 1

cur

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 51 / 59

Preceding Examples

Example (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/course[@id="4"]/semester/text()/preceding::node()

Result: 1 element node + 1 text node, root element is anscestor

/coursecatalog/course/preceding::node()

Result: the OOP course 4 element nodes + 3 text nodes

/coursecatalog/course[name="OOP"]/preceding::node()

Result: empty

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 52 / 59

Following Sibling

FindsAll siblings nodes following

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

cur 1 2 3

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 53 / 59

Following Sibling ExamplesExample (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/course/following-sibling::node()

Result: 1 element node (the DB course)

/coursecatalog/course[@id="2"]/following-sibling::node()

Result: empty

/coursecatalog/course/semester/following-sibling::node()

Result: 2 element nodes (descriptions)

/coursecatalog/course/@id/following-sibling::node()

Result: empty

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 54 / 59

Preceding Sibling

FindsAll siblings nodes before

Numbering

1

2

3 4

5

6

7

8 9

10 11

12 13 14

Example

2 1 cur

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 55 / 59

Preceding Sibling ExamplesExample (Document Tree)

/coursecatalog

course

id=4 name:OOP sem:3 dsc

course

id=2 name:DB sem:7 dsc

Queries/coursecatalog/course/preceding-sibling::node()

Result: 1 element node (the OOP course)

/coursecatalog/course[@id="2"]/preceding-sibling::node()

Result: 1 element node (the OOP course)

/coursecatalog/course/semester/preceding-sibling::node()

Result: 2 element nodes (names)

/coursecatalog/course/desc/preceding-sibling::node()

Result: 4 element nodes (0 attribute nodes)

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 56 / 59

Outline

1 Introduction

2 Tree Terminology

3 Location Path and Steps

4 XPath Path Expressions

5 Axes

6 Summary

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 57 / 59

Summary: XPath

Main PointsXPath is widely used

Not an XML syntax!

XPath is used for many purposes in related XML technologiesXQueryXSLTSQL/XML

W3C Recommendation November 1999 www.w3.org/TR/xpath

NoteVery good idea to get familiar with XPath

XPath is the foundation for understanding other XML technologies

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 58 / 59

Additional Information

Web Siteswww.w3schools.com/XPath/xpath_intro.asp: W3C is always agood place to start

www.stylusstudio.com/w3c/xpath/: A very good and quiteelaborated tutorial

www.devarticles.com/c/a/XML/Introduction-to-XPath/: Good4 page tutorial

pierre.senellart.com/wdmd/chap-xpath.pdf: A description ofthe XPath data model

Toolspgfearo.googlepages.com/: A very good tool for playing aroundwith XPath

There is an introduction screencast

http://www.bit-101.com/xpath/: A good online tool

Kristian Torp (Aalborg University) Introduction to XPath November 3, 2015 59 / 59