XQuery tutorial - basics
Introduction
This is a beginners' tutorial for XQuery.
Prerequisites:
- Basic knowledge of XML
- Some XPath. See the XPath tutorial - basics
Recommended reading, before this:
- XSLT Tutorial - Basics, in particular the short section on XPath.
- Basic knowledge of Xpath, see XPath tutorial - basics
- XML namespace (a little bit).
XQuery is an XML language for querying XML that can handle the following XML document types (both single documents or collections):
- files
- XML databases
- XML fragments in memory
XQuery can express queries across all these kinds of data, whether physically stored in XML or viewed as XML via middleware. In other words, you can use it to retrieve things from files, from XML representations made from SQL databases or from native XML databases like eXist. In addition, Xquery is quite a real programming language.
In more simple terms, the W3C XML Query Working Group advertized that “XQuery is a standardized language for combining documents, databases, Web pages and almost anything else. It is very widely implemented. It is powerful and easy to learn. XQuery is replacing proprietary middleware languages and Web Application development languages. XQuery is replacing complex Java or C++ programs with a few lines of code. XQuery is simpler to work with and easier to maintain than many other alternatives. Do more with less.”
XQuery:
- relies on the Xpath (version 2.0) language
- can generate new XML/HTML documents
XQuery does not define updating. You may use XQuery Update Facility, which is an XQuery extension. As of Jan 2010, this not a full recommendation, but it's implemented at least partially in some engines.
Standards:
- http://www.w3.org/TR/xquery/ (XQuery 1.0: An XML Query Language, W3C Recommendation 23 January 2007query language, stable)
- http://www.w3.org/TR/xquery-operators/ (XQuery 1.0 and XPath 2.0 Functions and Operators, W3C Recommendation 23 January 2007)
- http://www.w3.org/TR/xqupdate/ (XQuery Update Facility 1.0, W3C Candidate Recommendation 09 June 2009)
- XML Syntax for XQuery 1.0 (XQueryX) (barely eadable for humans, but practical for machine parsing of query statements).
- Authors of this standard: W3C XML Query Group
Use contexts
In order to play with XQuery, you need an XQuery processor. These are available in several forms:
- Included in some command line/library products like “saxon”. See the Shell script article for an examplantion on how to use it under Windows.
- Often, one of these processors is included in your XML editor, i.e. you should find XQuery processing in one of the Menu items.
- Typical XML databases include XQuery processors
- Some programming languages also include XQuery libraries. Otherwise they exist as external libraries for most programming languages.
In practical terms, XQuery files can be executed:
- From the command line
- Through an administration tool (e.g. a Web tool or a local XML editor)
- Through some web applications
Basic XQuery
Let's recall that XQuery uses XPath elements. Below are some simple examples that demonstrate this principle. Notice that these simple queries will not produce well-formed XML, but lists of well-formed XML fragments.
You should understand the following XPath expressions, else maybe read the XPath tutorial - basics again.
- Position in a tree
- ex: returns all nodes <card-propvalue> under <c3mssoft-desc> ...
//c3msbrick//c3mssoft-desc//card-propvalue
- Comparison
- ex: returns all nodes with this id == epbl
//c3msbrick//c3mssoft[@id="epbl"]
- Functions
- ex: title () only returns the content of a node
//c3msbrick/title/text()
Let's now examine a few simple XQuery examples that will use a for - return construct.
Find all nodes with path //topic/title
The following expression returns all fragments <topic><title>xxxx </title></topic> from anywhere in an XML document.
for $t in //topic/title
return $t
The next example shows how to find all nodes of //topic/title in a file called "catalog.xml". doc is a so-called XQuery 1.0 and XPath 2.0 function.
for $t in fn:doc("catalog.xml")//topic/title
return $t
The next example does the same, but retrieves the file via HTTP:
for $t in fn:doc("http://tecfa.unige.ch/proj/seed/catalog/net/xml/catalog-eng.xml")//topic/title
return $t
Result for all examples
<title>TECFA Seed Catalog</title>
<title>Introduction</title>
<title>Conceptual and technical framework</title>
<title>The socio-constructivist .... etc.
Wrapping a node list - creating well-formed XML
<result>
{ for $t in
fn:doc("http://tecfa.unige.ch/guides/xml/examples/shakespeare.1.10.xml/hamlet.xml")//ACT//SCENE/TITLE
return $t }
</result>
Result would look like this:
<?xml version="1.0" encoding="UTF-8"?>
<result>
<TITLE>SCENE I. Elsinore. A platform before the castle.</TITLE>
<TITLE>SCENE II. A room of state in the castle.</TITLE>
....
<result>
FLWOR expressions
Let's now introduce the core construct of a typical XQeury expression and which is known under the name FLOWR, which stands for "For-Let-Where-Order-Return"
This construct is similar to select-from-where-..-order-by that is used in SQL.
The FLOWR elements:
- for = iteration on a list of XML nodes
- let = binding of a result to a local variable
- where = selection condition
- order = sorting
- return = expression that will be returned
For :
for $variable in expression_search RETURN $variable
"for" associates to $variable each XML fragment found expression, defined as XPath expression.
Example:
for $t in //topic/title return $t
let:
- "let" assigns a value (node) to a variable
Example without let:
for $t in fn:doc("catalog09.xml")//c3msbrick//c3mssoft-desc//card-propvalue return $t
Same example with let
- for each node $t found, we bind $desc with a subnode.
for $t in fn:doc("catalog09.xml")//c3msbrick let $desc := $t//c3mssoft-desc//card-propvalue return $desc
Counting, for each node $t found in the "for" loop we count the number of subnodes:
for $t in fn:doc("catalog09.xml")//c3msbrick let $n := count($t//c3mssoft) return <result> {$t/title/text()} owns {$n} bricks </result>
where
- defines a selection condition
Same as above, but we only return results with a least 2 c3mssoft
for $t in fn:doc("catalog09.xml")//c3msbrick let $n := count($t//c3mssoft) where ($n > 1) return <result> {$t/title/text()} owns {$n} bricks </result>
Order:
- Can sort results
Example of an alphabetic sorting:
for $t in //topic/title order by $t return $t
More complex alphabetic sorting example:
for $t in fn:doc("catalog09.xml")//c3msbrick let $n := count($t//c3mssoft) where ($n > 1) order by $n return <result> {$t/title/text()} owns {$n} bricks </result>
Below is a slightly more complex return clause, we also include <titles> of <c3msbricks>:
for $t in fn:doc("catalog09.xml")//c3msbrick let $brick_softs := $t//c3mssoft let $n := count($brick_softs) where ($n > 0) order by $n return <result> For {$t/title/text()} we found {$n} softwares: {$brick_softs//title} </result>
- return builds the expression to return
- Warning: Each iteration must return a fragment, i.e. a single node (and not a collection!)
Good:
for $t in fn:doc("catalog09.xml")//c3msbrick let $n := count($t//c3mssoft) return <result> {$t/title/text()} owns {$n} bricks </result>
Bad:
for $t in fn:doc("catalog09.xml")//c3msbrick let $n := count($t//c3mssoft) return $t/title/text() owns $n bricks
Using XQuery
Creating well-formed XML result fragments
XML result of an XQuery usually should lead to a single node (not a list). Let's first look at a Multi-fragment version (collection):
for $t in //c3msbrick/title return $t
This will return a list (a so-called collection)
1 <title class ="- topic/title " > TECFA Seed Catalog </ title > 2 <title class ="- topic/title " > Introduction </ title > 3 <title class ="- topic/title " > Conceptual and technical framework </ title > 4 <title class ="- topic/title " > The socio-constructivist approach </ title > ....
This sort of list is not well-formed XML, and can not be displayed in a browser for example.
- ... but this is not a problem if is dealt with by some program (e.g. a php script querying a DOM tree)
Now let's see the single fragment version. The following expression:
<result>
{ for $t in //topic/title/text()
return <titre>{$t}</titre> }
</result>
returns:
<result >
<titre > TECFA Seed Catalog </ titre >
<titre > Introduction </ titre >
<titre > Conceptual and technical framework </ titre >
<titre > The socio-constructivist approach </ titre >
....
</result>
- We replaced "title" tags by "titre" tags (just to make this a bit more complicated)
- All expressions within { ...} are evaluated as XQuery.
- but all <tags> are copied as is ...
Multiple for loops
You can first have a look at the XML source file from which we shall retrieve data.
<result>
{ for $country in fn:doc ("http://www.dbis.informatik.uni-goettingen.de/Mondial/mondial.xml")/mondial/country,
$ethnicgroup in $country/ethnicgroups
return
<item>
<country> {$country/name/text()} </country>
<ethnicgroup> {$ethnicgroup/@percentage} {$ethnicgroup/text()}</ethnicgroup>
</item>
}
</result>
The result is maybe not what you like (each country can appear more than once)
<result>
<item><country>Albania</country><ethnicgroup percentage="3">Greeks</ethnicgroup></item>
<item><country>Albania</country><ethnicgroup percentage="95">Albanian</ethnicgroup></item>
<item><country>Greece</country><ethnicgroup percentage="98">Greek</ethnicgroup></item>
So here is another example that creates items per country.
<result>
{ for $country in fn:doc ("http://www.dbis.informatik.uni-goettingen.de/Mondial/mondial.xml")/mondial/country
return
<item>
<country> {$country/name/text()} </country>
{ for $ethnicgroup in $country/ethnicgroups
return
<ethnicgroup> {$ethnicgroup/@percentage} {$ethnicgroup/text()}</ethnicgroup>
}
</item>
}
</result>
Result looks like:
<result>
<item>
<country>Albania</country>
<ethnicgroup percentage="3">Greeks</ethnicgroup>
<ethnicgroup percentage="95">Albanian</ethnicgroup>
</item>
<item>
<country>Greece</country>
<ethnicgroup percentage="98">Greek</ethnicgroup></item>
<item>
<country>Macedonia</country>
<ethnicgroup percentage="22">Albanian</ethnicgroup>
<ethnicgroup percentage="2">Serb</ethnicgroup>
<ethnicgroup percentage="65">Macedonian</ethnicgroup>
<ethnicgroup percentage="4">Turkish</ethnicgroup>
<ethnicgroup percentage="3">Gypsy</ethnicgroup>
</item>
Here is another example file to query (made by the author some years ago). You may look at it before you study the xquery example code.
The following example shows how to generate some text that includes a count variable:
<result>
<title>List of C3MSBricks and associated Software</title>
{ for $t in fn:doc("http://tecfa.unige.ch/proj/seed/catalog/net/xml/catalog-eng.xml")//c3msbrick
let $brick_softs := $t//c3mssoft
let $n := count($brick_softs)
where ($n > 0)
order by $n descending
return
<brick> Pour {$t/title/text()} on a les {$n} modules suivants:
{ for $soft in $brick_softs
return <soft> {$soft//title/text()}</soft>
}
</brick>
}
</result>
Some explanations / recalls:
- Each FLWOR expression must be within { ... }
- The "return" clause of the outer loop includes a loop that will deal with
- $brick_softs contains a collection of $softs from which we extract titles
- We also sort the results
Result:
<result>
<title>List of C3MSBricks and associated Software</title>
<brick> Pour User statistics on a les 6 modules suivants:
<soft>pnProdAct</soft>
<soft>commArt</soft>
<soft>pncUserPoints</soft>
<soft>pncSimpleStats </soft>
<soft>Statistics module</soft>
<soft>NS-User_Points</soft>
</brick>
<brick> Pour Gallery on a les 5 modules suivants:
<soft>PhotoShare</soft>
<soft>Photoshare combined with PageSetter</soft>
<soft>My_eGallery</soft>
<soft>Coppermine </soft>
<soft>Gallery </soft>
</brick>
.....
</result>
XHTML output
Let's pull in XML files, extract some informations from it and render the result in HTML.
xquery version "1.0";
let $source_doc := fn:doc("http://tecfa.unige.ch/guides/xml/examples/shakespeare.1.10.xml/hamlet.xml")
return
<html>
<head> <title>Questionnaire Items</title> </head>
<body>
Acts of Hamlet
<ol>
{ for $t in $source_doc//ACT//SCENE/TITLE
return
<li> {$t/text()} </li>
}
</ol>
</body>
</html>
This would produce something like:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Questionnaire Items</title>
</head>
<body>
Acts of Hamlet
<ol>
<li>SCENE I. Elsinore. A platform before the castle.</li>
<li>SCENE II. A room of state in the castle.</li>
<li>SCENE III. A room in Polonius' house.</li>
<li>SCENE IV. The platform.</li>
<li>SCENE V. Another part of the platform.</li>
<li>SCENE I. A room in POLONIUS' house.</li>
<li>SCENE II. A room in the castle.</li>
<li>SCENE I. A room in the castle.</li>
.....
</ol>
</body>
</html>
The next example is somewhat inspired by XQuery/Displaying data in HTML Tables of the nice XQuery wikibook.
It's the equivalent of the example discusses in the PHP - MySQL - XML tutorial - basics tutorial. Instead of using XSLT to format a typical database result table, we use XQuery.
XML must have the following structure: <result><row>....</row> <row>....</row> </result>. Inside the row element, you may have any number of XML elements.
xquery version "1.0";
let $xml_source_doc :=
fn:doc('http://tecfa.unige.ch/guides/xml/examples/mysql-php-xml/testfile.xml')
return
<html>
<head> <title>Questionnaire Items</title> </head>
<body>
This example shows how to render a table-like XML structure as HTML table.
<table>
<thead>
<tr>
{ let $a_row := $xml_source_doc/result/row[1]
for $element in $a_row/*
return
<th>{ fn:node-name($element) }</th>
}
</tr>
</thead>
<tbody>
{
for $row at $count in $xml_source_doc/result/row
return
<tr>
{ if ($count mod 2) then (attribute {'bgcolor'} {'Lavender'}) else () }
{ for $cell in $row/*
return
<td>{ $cell }</td>
}
</tr>
}
</tbody>
</table>
</body>
</html>
A complete example
This example has been made by Daniel Dean, exchange student at Webster Geneva, spring 1 2008.
Consider the following XML:
<?xml version="1.0"?>
<catalog>
<book id="bk101">
<author>Gambardella, Matthew</author>
<title>XML Developer's Guide</title>
<genre>Computer</genre>
<price>44.95</price>
<publish_date>2000-10-01</publish_date>
<description>An in-depth look at creating applications
with XML.</description>
</book>
<book id="bk102">
<author>Ralls, Kim</author>
<title>Midnight Rain</title>
<genre>Fantasy</genre>
<price>5.95</price>
<publish_date>2000-12-16</publish_date>
<description>A former architect battles corporate zombies,
an evil sorceress, and her own childhood to become queen
of the world.</description>
</book>
<book id="bk103">
<author>Corets, Eva</author>
<title>Maeve Ascendant</title>
<genre>Fantasy</genre>
<price>5.95</price>
<publish_date>2000-11-17</publish_date>
<description>After the collapse of a nanotechnology
society in England, the young survivors lay the
foundation for a new society.</description>
</book>
<book id="bk104">
<author>Corets, Eva</author>
<title>Oberon's Legacy</title>
<genre>Fantasy</genre>
<price>5.95</price>
<publish_date>2001-03-10</publish_date>
<description>In post-apocalypse England, the mysterious
agent known only as Oberon helps to create a new life
for the inhabitants of London. Sequel to Maeve
Ascendant.</description>
</book>
<book id="bk105">
<author>Corets, Eva</author>
<title>The Sundered Grail</title>
<genre>Fantasy</genre>
<price>5.95</price>
<publish_date>2001-09-10</publish_date>
<description>The two daughters of Maeve, half-sisters,
battle one another for control of England. Sequel to
Oberon's Legacy.</description>
</book>
<book id="bk106">
<author>Randall, Cynthia</author>
<title>Lover Birds</title>
<genre>Romance</genre>
<price>4.95</price>
<publish_date>2000-09-02</publish_date>
<description>When Carla meets Paul at an ornithology
conference, tempers fly as feathers get ruffled.</description>
</book>
<book id="bk107">
<author>Thurman, Paula</author>
<title>Splish Splash</title>
<genre>Romance</genre>
<price>4.95</price>
<publish_date>2000-11-02</publish_date>
<description>A deep sea diver finds true love twenty
thousand leagues beneath the sea.</description>
</book>
<book id="bk108">
<author>Knorr, Stefan</author>
<title>Creepy Crawlies</title>
<genre>Horror</genre>
<price>4.95</price>
<publish_date>2000-12-06</publish_date>
<description>An anthology of horror stories about roaches,
centipedes, scorpions and other insects.</description>
</book>
<book id="bk109">
<author>Kress, Peter</author>
<title>Paradox Lost</title>
<genre>Science Fiction</genre>
<price>6.95</price>
<publish_date>2000-11-02</publish_date>
<description>After an inadvertant trip through a Heisenberg
Uncertainty Device, James Salway discovers the problems
of being quantum.</description>
</book>
<book id="bk110">
<author>O'Brien, Tim</author>
<title>Microsoft .NET: The Programming Bible</title>
<genre>Computer</genre>
<price>36.95</price>
<publish_date>2000-12-09</publish_date>
<description>Microsoft's .NET initiative is explored in
detail in this deep programmer's reference.</description>
</book>
<book id="bk111">
<author>O'Brien, Tim</author>
<title>MSXML3: A Comprehensive Guide</title>
<genre>Computer</genre>
<price>36.95</price>
<publish_date>2000-12-01</publish_date>
<description>The Microsoft MSXML3 parser is covered in
detail, with attention to XML DOM interfaces, XSLT processing,
SAX and more.</description>
</book>
<book id="bk112">
<author>Galos, Mike</author>
<title>Visual Studio 7: A Comprehensive Guide</title>
<genre>Computer</genre>
<price>49.95</price>
<publish_date>2001-04-16</publish_date>
<description>Microsoft Visual Studio 7 is explored in depth,
looking at how Visual Basic, Visual C++, C#, and ASP+ are
integrated into a comprehensive development
environment.</description>
</book>
</catalog>
Below are three different XQuery code snippets. You can test these with the Firefox XQuery USE ME (XqUSEme) add-on (install it first if necessary).
- Open books.xml in the Firefox browser
- Select Tools > Perfom XQuery
- Under the XQuery Tab, ensure that "Use this document for query?" is checked
- Copy and paste 1 of the 3 XQuery commands [see below] in the XQuery textarea
- On the right, select "Open in a new tab" AND change "output file ext."
- Select "Perform XQuery" and view the results
Xquery 1:
xquery version "1.0";
<html>
<head>
<title>Danny Dean - XQuery 1</title>
</head>
<body>
<h1>Book Titles (A-Z)</h1>
<div>
<ul>
{
for $b in doc()//catalog/book
order by $b/title ascending
return
<li>{$b/title/text()}</li>
}
</ul>
</div>
</body>
</html>
Xquery 2:
xquery version "1.0";
<html>
<head>
<title>Danny Dean - XQuery 2</title>
</head>
<body>
<h1>Books by Genre</h1>
<div>
{
for $genre in fn:distinct-values(doc()//catalog/book/genre)
return
<div>
<h3>{$genre}</h3>
<ul>
{
for $b in doc()//catalog/book
where $b/genre = $genre
return
<li>{$b/title/text()}</li>
}
</ul>
</div>
}
</div>
</body>
</html>
Xquery 3:
xquery version "1.0";
<html>
<head>
<title>Danny Dean - XQuery 3</title>
</head>
<body>
<h1>Most and Least Expensive Books</h1>
{
let $books := doc()//catalog/book
let $max := $books[price = max($books/price)]
let $min := $books[price = min($books/price)]
return
<div>
<div>Most Expensive: {$max[1]/title/text()} - ${$max[1]/price/text()}</div>
<div>Least Expensive: {$min[1]/title/text()} - ${$min[1]/price/text()}</div>
</div>
}
</body>
</html>
Declarations
This section just shows some more advanced features. XQuery is a full programming language (stuff for another tutorials....).
- Namespaces
If an XML file has namespaces, they must be declared. If there is a single, then it's easiest to declare it as default namespace, e.g.
xquery version "1.0";
declare default element namespace 'http://www.w3.org/1999/xhtml';
Here is an example that deals with XML that includes RDF and Dublin Core metadata
xquery version "1.0";
declare namespace rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#";
declare namespace dc="http://purl.org/dc/elements/1.1/";
If there are more than one, e.g. an XHTML with some XML inside, then you'd have to declare the namespace plus use a prefix within the queries (example taken from the XqUSeme Firefox extension help)
declare namespace html = 'http://www.w3.org/1999/xhtml';
<textarea rows='20' cols='50'> {
for $par in doc()//html:p
where starts-with($par, 'This')
return <p>{$par/string()}</p> } </textarea>
- Proprietary extensions
- Most vendors implement various extensions, e.g. declarations for HTML serialization of output.
- Variables (untyped and typed)
declare variable $x := 7.5;
declare variable $x as xs:integer := 7;
- Functions
“XQuery allows users to declare functions of their own. A function declaration specifies the name of the function, the names and datatypes of the parameters, and the datatype of the result. [..] A function declaration specifies whether a function is user-defined or external. For a user-defined function, the function declaration includes an expression called the function body that defines how the result of the function is computed from its parameters.” XQuery 1.0: An XML Query Language (specification, , retrieved 21:13, 11 February 2010 (UTC)).
The following example is also taken from the specification:
declare function local:summary($emps as element(employee)*)
as element(dept)*
{
for $d in fn:distinct-values($emps/deptno)
let $e := $emps[deptno = $d]
return
<dept>
<deptno>{$d}</deptno>
<headcount> {fn:count($e)} </headcount>
<payroll> {fn:sum($e/salary)} </payroll>
</dept>
};
local:summary(fn:doc("acme_corp.xml")//employee[location = "Denver"])
The function call is in the last line.
Links
Sample XML files on-line
(that you may use for playing with XQuery)
- Shakespeare's play have been translated to XML by Jon Bozak. The original is at Oasis (Zip), but they also can be found on the Internet. E.g. search for hamlet.xml. Café con Leche has a copy
- The Mondial Database (Institute for Informatics,Georg-August-Universität Göttingen). E.g. use
- http://www.dbis.informatik.uni-goettingen.de/Mondial/mondial.xml (a database with demographics data about countries)
- Dive Into Python
- XML (DocBook) (400 page DocBook example) as zip file.
Software and standards
See the XQuery article.
According to the W3C, there are over 40 different software packages that support XML Query in some way. However, this does not mean that non-programmers' XQuery tools can be easily found currently (Jan 2009).
An easy way to play with XQuery is to install a browser extension. We tried out:
- XQuery USE ME (XqUSEme) which is based on the (serious) Saxon B engine. The only problem I had is that it will try to validate and if the DTD is missing then there is trouble.
XQuery Tutorials
See also the XQuery article
- XQuery (Wikipedia article)
- XQuery - Wikibook
- XQuery Tutorial (W3C Schools)
- Essential XQuery - The XML Query Language by Darshan Singh. (Feb 2, 2004). Good !
- XQuery Tutorial by P. Fankhauser and P. Wadler.
- An introduction to XQuery, IBM Developer works (2002, updated 2006).
- XQuery, tutorial by Anders Möller & Michael I. Schwartzbach [2003]. See also:
- Chapter 6 - Querying XML Documents with XQuery (slides) and their book An Introduction to XML and Web Technologies, ISBN 0321269667
- XQuery Tutorial from IPEDO. Quite technical.
- XQuery from the Experts: Influences on the design of XQuery Webreference Article, excerpt is from Chapter 2 of the Addison-Wesley book XQuery from the Experts. [2003]
- Michael Brundage (2004). XQuery: The XML Query Language, Portland: Book News [this is a good book]
- Comparing XSLT and XQuery by Michael Kay, Saxonica (not really a tutorial, but useful)
- Learn XQuery in 10 Minutes: An XQuery Tutorial and Blooming FLWOR - An Introduction to the XQuery FLWOR Expression by Michael Kay (but centered on a commercial editor)
- From Social to Serious: Combining Twitter and RESTful Web Services by Curt Cagle, DevX.com, March 20, 2009.
- XQuery: A Guided Tour - Download the free tutorial today. This is an 80-page tutorial from the XQuery from the Experts: A Guide to the W3C XML Query Language book by Howard Katz, Don Chamberlin, Denise Draper, Mary Fernandez, Michael Kay, Jonathan Robie, Michael Rys, Jerome Simeon, Jim Tivy, and Philip Wadler, (ISBN 0-321-18060-7), 2004.
XQuery Web sites
- DataDirect's XQuery Developer Center (tutorials, examples, software)
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