XPath and XQuery - Search - University of Malta

XPath and XQuery

Charlie AbelaDepartment of Artificial Intelligence

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mailto:[email protected]

CSA 3210 XPath & XQuery © 2

Last Lecture

Introduction to XMLUse of DTD and XML schema to validate XML

Problems?


Lecture Outline

XPath: a practical language to select nodes from an XML documentXQuery: compatible with XPath and is used for finding and extracting elements and attributes from XML documents. Exercise


Querying XML

In relational databases, parts of a database can be selected and retrieved using SQL

Same necessary for XML documentsEssentially moving from relations to treesQuery languages: XQuery, XQL, XML-QL

The central concept of XML query languages is a path expression

Specifies how a node or a set of nodes, in the tree representation of the XML document can be reached


XPath

XPath is a language for addressing parts of an XML document.

It operates on the tree data model of XMLIt has a non-XML syntax

Two types of Path ExpressionsAbsolute (starting at the root of the tree)

Syntactically they begin with the symbol /It refers to the root of the document (situated one level above the root element of the document)

Relative to a context node


Path through Tree Model

Finding all the books with a title “Artificial Intelligence”

Library Example<library location="Bremen">

<author name="Henry Wise"><book title="Artificial Intelligence" ISBN="12345678"/><book title="Modern Web Services" ISBN="87654"/><book title="Theory of Computation" ISBN="2468"/>

</author><author name="William Smart">

<book title="Artificial Intelligence" ISBN="1357"/></author><author name="Cynthia Singleton">

<book title="The Semantic Web" ISBN="5678"/><book title="Browser Technology Revised" ISBN="12345"/>

</author></library>


General Form of Path Expressions

A path expression consists of a series of steps, separated by slashes

/library/author/book

A step consists of An axis specifier, A node test, and An optional predicate

//author[1]/book[1]axis specifier node test

predicates


General Form of Path Expressions (2)

An axis specifier determines the tree relationship between the nodes to be addressed and the context node

E.g. parent, ancestor, child (the default), sibling, attribute node// is such an axis specifier: descendant or self

//child::author/attribute::name//author/@name



A node test specifies which nodes to address

The most common node tests are element names E.g., * addresses all element nodescomment() addresses all comment nodes

//author/*



Predicates (or filter expressions) are optional and are used to refine the set of addressed nodes

E.g., the expression [1] selects the first node[position()=last()] selects the last node[position() mod 2 =0] selects the even nodes


Path Expressions in XPath (1)

Address all author elements/library/author

Addresses all author elements that are children of the library element node, which resides immediately below the root


Result of Query (1)



Address all author elements//author

Here // says that we should consider all elements in the document and check whether they are of type authorThis path expression addresses all authorelements anywhere in the document


Result for Query (2)



Address the location attribute nodes within library element nodes

/library/@location

The symbol @ is used to denote attribute nodes


Result of Query 3



Select all title attribute nodes within bookelements anywhere in the document, which have the value “Artificial Intelligence”

//book/@title="Artificial Intelligence"



Select all books with title “Artificial Intelligence”

//book[@title="Artificial Intelligence"]

Test within square brackets: a filter expressionIt restricts the set of addressed nodes.

Difference with query 4. Query 5 filters all book elements, the title of which satisfies a certain condition.Query 4 checks for certain title attribute nodes of bookelements


Result of Query (4)


Result of Query (5)



Address the first author element node in the XML document//author[1]

Address the last book element within the first author element node in the document//author[1]/book[last()]

Select all book element nodes whose title is not “Artificial Intelligence”//book[@title!=“Artificial Intelligence”]


Results of Query set (6)



Count the number of authorscount(//author)

List the books written by “Henry Wise”//author[@name="Henry Wise"]/book

Count the number of books written by “William Smart”count(//author[@name="William Smart"]/book)


Results of Query set (7)

XPath embedded in JSPs<%@ taglib uri="/src/tags/taglib.tld" prefix="mytag" %><html> <head>

<title>View Orders</title> </head> <body bgcolor="white" link="#666699" vlink="#666699"> OrderDate: <mytag:getvalue select=“/ORDER/DATE" /> <HR>  <mytag:ifdef select=“/ORDER/SHIPTO" > ShipTo: <mytag:getvalue select=“ /ORDER/SHIPTO/NAME " /> <mytag:getvalue select=“ /ORDER/SHIPTO/STREET " /> <mytag:getvalue select=“ /ORDER/SHIPTO/CITY " /> <mytag:getvalue select=“ /ORDER/SHIPTO/STATE " /> <mytag:getvalue select=“ /ORDER/SHIPTO/ZIP " /> 

</mytag:ifdef>


What is XQuery?

A query language that allows you to:select elements/attributes from input documentsjoin data from multiple input documentsmake modifications to the datacalculate new dataadd new elements/attributes to the resultssort your results

Slides adapted from presentation by Priscilla Walmsleyhttp://www.datypic.com/services/xquery/intro.html


Example


Uses of XQuery

As varied as the uses for XML. Examples:Extracting information from a database for use in a Web serviceGenerating summary reports on data stored in an XML databaseSearching textual documents on the Web for relevant information and compiling the resultsSelecting and transforming XML data to XHTML to be published on the WebPulling data from databases to be used for application integrationSplitting up an XML document that represents multiple transactions into multiple XML documents


XQuery Processing


XML Input

Could be:text files that are XML documentsfragments of XML documents that are retrieved from the web using a URIa collection of XML documents that are associated with a particular URIdata stored in native XML databasesdata stored in relational databases that have an XML front-endin-memory XML documents


Example: Catalogue.xml


Selecting nodes


Results


Path Expressions and FLWOR

•FLWOR expressions are analogous to SQL Select statement•FLWOR is an acronym for "For, Let, Where, Order by, Return".


Sort results

Wrap results in a <ul> element

Wrap each name in a <li> element


Eliminate the name


Tools

XPath Evaluator (free online):http://b-cage.net/code/web/xpath-evaluator.html

XQueryXMLSpy (trial): http://www.altova.comStylus Studio (trial): http://www.stylusstudio.com/


XPath ExerciseDownload the file bibliography.xml (from course material page)Write XPath queries for the following:

the titles of articles the second article the second author of the second article the authors of the article whose title is 'Safe Constraint Queries' the titles of articles where 'Frank Neven' is one of the authors the titles of articles where both 'Frank Neven' and 'Dirk Van Gucht' are authors the titles of articles where either 'Frank Neven' or 'Dirk Van Gucht' is an author the number of articles the titles of articles with only one author the number of articles with more than three authors

Check your queries against XPath Evaluator or some other tool


Suggested Reading

Semantic Web Primer, Chapter 2XML Path Language, J.Clark, http://www.w3.org/TR/xpathXPath tutorial: http://www.w3schools.com/xpath/default.aspXQuery tutorial: http://www.w3schools.com/xquery/default.aspDetailed XQuery tutorial: http://www.datypic.com/services/xquery/IntroductionToXQuery.pdf


Next Lecture

Introduction to RDF and RDFSCore concepts of the languagesExamples

<weather time="2002-06-06T15:35:00-05:00"> <report latitude="41.2° N" longitude="71.6° W"> <locality>Block Island</locality> <temperature units="°C">16</temperature> <humidity>88%</humidity> <dewpoint units="°C">14</dewpoint> <wind>

<direction>NE</direction> <speed units="km/h">16.1</speed> <gust units="km/h">31</gust>

</wind> <pressure units="hPa">1014</pressure><condition>overcast</condition> <visibility>13 km</visibility>

</report> <report latitude="34.1° N" longitude="118.4° W"> <locality>Santa Monica</locality> <temperature units="°C">19</temperature> <humidity>79%</humidity> <dewpoint units="°C">16</dewpoint> <wind>

<direction>WSW</direction> <speed units="km/h">14.5</speed>

</wind> <pressure units="hPa">1010</pressure><condition>hazy</condition> <visibility>5 km</visibility>

</report> </weather>


QueriesSelect the report elements.

/weather/report

Select the first report element. /weather/report[1]

Select the temperature elements/weather/report/temperature

Select the report element whose locality is “Santa Monica”/weather/report[locality="Santa Monica"]

Select the longitude attribute of the first report element//report[locality="Block Island"]/attribute::longitudeSelect all the direction, speed, and gust elements/child::weather/child::report/child::wind/child::*Return the temperature on “Block Island” in degrees Fahrenheit9 * number(/weather/report[locality="Block Island"]/temperature) div 5 + 32

Select all the elements in the document./descendant::*

XPath and XQuery - Search - University of Malta

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