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CS 253: Topics in Database Systems: C2. Dr. Alexandra I. Cristea http://www.dcs.warwick.ac.uk/~acristea/. Previously we looked at: XML XSL XSLT Next: XPath XQuery. XPath. XPath. XPath is a syntax for defining parts of an XML document

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CS 253: Topics in Database Systems: C2

Dr. Alexandra I. Cristea


  • Previously we looked at:

    • XML

    • XSL

    • XSLT

  • Next:

    • XPath

    • XQuery



  • XPath is a syntax for defining parts of an XML document

  • XPath uses path expressions to navigate in XML documents

  • XPath contains a library of standard functions

  • XPath is a major element in XSLT

  • XPath is a W3C recommendation, thus a Standard (16. November 1999 )

XPath Path Expressions

  • Uses path expressions to select nodes or node-sets in an XML document.

    • These path expressions look very much like the expressions you see when you work with a traditional computer file system.

XPath Standard Functions

  • over 100 built-in functions.

    • string values,

    • numeric values,

    • date and time comparison,

    • node and QName manipulation,

    • sequence manipulation,

    • Boolean values,

    • and more.

XPath Terminology

  • Nodes

  • Atomic values

  • Items (atomic values or nodes)

  • Relationships of nodes

    • Parent

    • Children

    • Siblings

    • Ancestors

    • Descendants

XPath Nodes

  • 7 kinds of nodes:

    • element,

    • attribute,

    • text,

    • namespace,

    • processing-instruction,

    • comment, and

    • document (root) nodes.

      • XML documents are treated as trees of nodes. The root of the tree is called the document node (or root node).

Document node

Element node

Attribute node

Nodes Examples

<?xml version="1.0" encoding="ISO-8859-1"?>



<title lang="en">Harry Potter</title>

<author>J K. Rowling</author>





Atomic values Examples*

<?xml version="1.0" encoding="ISO-8859-1"?>



<title lang="en">Harry Potter</title>

<author>J K. Rowling</author>





*nodes with no children or parent

Selecting nodes

Examples of selecting nodes


  • Predicates are used to find a specific node or a node that contains a specific value.

  • Predicates are always embedded in square brackets.

Example predicates

Example predicates – cont.

Selects all the book elements of the

bookstore element that have a price

element with a value greater than 35.00

Selects all the title elements of the

book elements of the bookstore

element that have a price element

with a value greater than 35.00

Selecting Unknown Nodes

Example: selecting several paths

Selects all the title AND price elements in the document

Selects all the title elements of the book element of the bookstore element AND all the price elements in the document

Location Path Expression

  • A location path can be absolute or relative.

  • An absolute location path: /step/step/...

  • A relative location path: step/step/...

  • Location step:


XPath Axes


  • //DDD/parent::*

    <AAA>  <BBB>               <DDD>

                   </DDD> </BBB>



  • //BBB/child::*

    <AAA> <BBB>  <DDD>

                   </DDD>           </BBB>


Note: /AAA is equivalent to /child::AAA

More examples

  • http://www.zvon.org/xxl/XPathTutorial/General/examples.html

    • Check basics, //, *, predicates, attributes, functions (new ones: count, name, normalize-space, starts-with, contains, string-length, floor, ceiling), axes, operators (mod)

    • Note: The ancestor, descendant, following, preceding and self axes partition a document (ignoring attribute and namespace nodes): they do not overlap and together they contain all the nodes in the document. (see example)

XPath Conclusion

  • We have learned:

    • XPath definition

    • Path expressions

    • Standard functions

    • Terminology

    • Predicates

    • Location paths

    • Axes

    • Some operators

  • Before we go on, one more thing about XML:

  • XML Namespaces

Naming ambiguity

The Idea to Solve it

  • Assign a URI (~ URL) to every sub-language:

    • E.g., for XHTML 1.0: http://www.w3.org/1999/xhtml

  • Qualify element names with URIs:

    • {http://www.w3.org/1999/xhtml}head

Web Naming and Addressing Overview (URIs, URLs, ...)

The actual solution

  • Namespace declarations bind URIs to prefixes:

  • Default namespace (no prefix) declared with: xmlns=“…”

  • Lexical Scope

  • Attribute names can also be prefixed

Applying namespaces

  • Next we look at how to query XML

  • This can be done, to some extent, as we have seen, within XSLT,

  • but the main language developed for this purpose is …


What is XQuery?

  • XQuery is the language for querying XML data

  • XQuery for XML is like SQL for databases

  • XQuery is built on XPath expressions

  • XQuery is defined by the W3C

  • XQuery is supported by all the major database engines (IBM, Oracle, Microsoft, etc.)

  • XQuery is a W3C recommendation (Jan 2007) thus a standard

XQuery and XPath

  • XQuery 1.0 and XPath 2.0 share the same data model and support the same functions and operators.

XQuery - Examples of Use

  • Extract information to use in a Web Service

  • Generate summary reports

  • Transform XML data to XHTML

  • Search Web documents for relevant information

Usage Scenario: Document-Oriented

  • Queries could be used

    • To retrieve parts of documents

    • To provide dynamic indexes

    • To perform context-sensitive searching

    • To generate new documents as combinations of existing ones

Usage Scenario: Programming

  • Queries could be used to automatically generate documentation

Usage Scenario: Hybrid

  • Queries could be used to data mine hybrid data, such as patient records

XQuery compared to XPath

  • XQuery 1.0 is a strict superset of XPath 2.0

  •  XPath 2.0 expression is directly an XQuery 1.0 expression (a query)

  • The extra expressive power is the ability to:

    • Join information from different sources and

    • Generate new XML fragments

Relationship to XSLT

  • XQuery, XSLT: both domain-specific languages for combining and transforming data from multiple sources

  • different in design - historical reasons

    • XQuery: designed from scratch

    • XSLT: intellectual descendant of CSS

  • technically, they may emulate each other

XQuery query

  • Prolog

    • Like XPath, XQuery expressions are evaluated relatively to a context

    • explicitly provided by a prolog (header)

      ~ header with definitions

  • Body

    • The actual query

XQuery Prolog (i.e., header(s))

  • Settings define various parameters for the XQuery processor language, such as:

    xquery version “1.0”;

    module "http://www.w3.org/2003/05/xpath-functions"

    default element namespace= "http://www.w3.org/1999/xhtml"

    declare namespace xs= "http://www.w3.org/2001/XMLSchema"

    import module "http://www.w3.org/2003/05/xpath-functions" at "logo.xq"

    define function addLogo($root as node()) as node()* { }

    (: etc :)

XQuery capabilities

  • Generate

  • Join

  • Select

Generate: constructors

  • XQuery expressions may compute new XML nodes

  • Expressions may denote:

    • element, character data, comment and processing instruction nodes

  •  node is created with a unique node identity

  • Constructors may be either

    • direct or

    • computed

Direct constructors in XQuery

<XMLfragment>my fragment </XMLfragment>

  • Evaluates to the given XML fragment

Explicit, computed constructors

Variable bindings

<employee empid="{$id}"> <name>{$name}</name>

{$job} <deptno>{$deptno}</deptno> <salary>{$SGMLspecialist+100000}</salary>


How to Select Nodes with XQuery?

  • Functions

    • XQuery uses functions to extract data from XML documents.

  • (X)Path Expressions

    • XQuery uses path expressions to navigate through elements in an XML document.

  • Predicates

    • XQuery uses predicates to limit the extracted data from XML documents.


  • doc()

    • function to open a file

  • Example:

    • doc("books.xml")

  • Note: A call to a function can appear where an expression may appear.

Path Expressions

  • Example:

    select all the title elements in the "books.xml" file:



  • Example:

    select all the book elements under the bookstore element that have a price element with a value that is less than 30 :


At a glance: function, path, predicate


  • For, Let, Where, Order by, Return

    = main engine

    ~ SQL syntax (SFHW)

    ~ programs and function calls

FLWOR by comparison with Path expressions

  • select all the title elements under the book elements that are under the bookstore element that have a price element with

    a value that is higher than 30.

  • Path expression:


  • FLWOR expression:

    for $x in doc("books.xml")/bookstore/book

    where $x/price>30

    return $x/title

Sorting in FLWOR

  • for $x in doc("books.xml")/bookstore/book

    where $x/price>30

    order by $x/title

    return $x/title

Present the Result In an HTML List



for $x in doc("books.xml")/bookstore/book/title

order by $x

return <li>{$x}</li>



Result HTML List


<li><title lang="en">Everyday Italian</title></li>

<li><title lang="en">Harry Potter</title></li>

<li><title lang="en">Learning XML</title></li>

<li><title lang="en">XQuery Kick Start</title></li>


Eliminate element (here: title)



for $x in doc("books.xml")/bookstore/book/title

order by $x

return <li>data($x)</li> (: also text() :)



New result HTML List


<li>Everyday Italian</li>

<li>Harry Potter</li>

<li>Learning XML</li>

<li>XQuery Kick Start</li>


Another FLWOR Expression

The Difference between for and let

The Difference between for and let

The Difference between for and let

The Difference between for and let

FLWOR Basic Building Blocks

General rules

  • for and let may be used many times in any order

  • only one where is allowed

  • many different sorting criteria can be specified (descending, ascending, etc.)

Joining documents

for $p IN doc("www.irs.gov/taxpayers.xml")//person

for $n IN doc("neighbors.xml")//neighbor[ssn = $p/ssn]



<ssn> { $p/ssn } </ssn>

{ $n/name }

<income> { $p/income } </income>



FOR $b IN doc(“bib.xml”)/bookRETURN <short>   {$b/title}   <author>    {IF count($b/author) < 3      $b/authorELSE      $b/author[1], <author>and others</author>    }   </author> </short>


  • DEFINE FUNCTIONdepth($e) RETURNS xsd:integer{ IF (empty($e/*) THEN 1 ELSE max(FOR $c in $e/* RETURN depth($c)) ) +1}

  • FOR $b in doc(“bib.xml”)/bookRETURN depth($b)

Existential and Universal Quantifiers

  • FOR $b in doc(“bib.xml”)/bookWHERE SOME $author IN $b/author   SATISFIES $author/text() = “Ullman”RETURN $b

  • FOR $b in doc(“bib.xml”)/bookWHERE EVERY $author IN $b/author           SATISFIES $author/text() = “Ullman”RETURN $b

Return books where at least one author is “Ullman”

Return books where all authors are “Ullman”

XQuery on Distributed Sources

XQuery Syntax

  • Declarative, functional language

    ~ SQL

  • Nested expressions

  • Case sensitive

  • White spaces:

    • Tabs, space, CR, LF

    • Ignored between language constructs

    • Significant in quoted strings

  • No special EOL character

Keywords and names

  • Keywords and operators

    • Case-sensitive, generally lower case

    • May have several meanings depending on the context

      • E.g. “*” or “in”

    • No reserved words

  • All names must be valid XML names

    • For variables, functions, elements, attributes

    • Can be associated with a namespace



  • Value comparisons

    Eq, ne, lt, le, gt, ge

    Used to compare individual values

    Each operand must be a single atomic value (or a node containing a single atomic value)

  • General comparisons

    =, !=, <, <=, >, >=

    Can be used with sequences of multiple items


Query Prolog

XQuery gives you a choice:

  • Path Expressions:

    • If you just want to copy certain elements and attributes as is

  • FLWOR Expressions:

    • Allow sorting

    • Allow adding elements/attributes

    • Verbose, but can be clearer

XQuery tools

  • XStylus Studio 2007 http://www.stylusstudio.com/xml_download.html (free trial version)

    • See also short its XQuery intro at: http://www.stylusstudio.com/xquery_primer.html

XML and programming

  • XSLT, XPath and XQuery provide tools for specialized tasks.

  • But many applications are not covered:

    • domain-specific tools for concrete XML languages

    • general tools that nobody has thought of yet

XML in general-purpose programming languages

  • parse XML documents into XML trees

  • navigate through XML trees

  • construct XML trees

  • output XML trees as XML documents

  • DOMand SAX are corresponding APIs that are language independent and supported by numerous languages. JDOM is an API that is tailored to Java.

XQuery Conclusion

  • We have learned:

    • XQuery definition

    • Usage scenarios

    • Comparison w. XSLT and XPath

    • Capabilities

    • Functions, path expressions and predicates

    • FLWOR

    • Extensions for generic programming with XML

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