XQuery tutorial - basics

Draft

Introduction

This is a beginners tutorial for XQuery.

Prerequisites:

• Basic knowledge of XML
• Some XPath

Recommended reading, before this:

• XSLT Tutorial - Basics, in particular the short section on XPath.
• Basic knowledge of Xpath, see XPath tutorial - basics
• XML namespace

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 (e.g. DOM trees)

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, from native XML databases like eXist. In addition, Xquery is quite a real programming language

XQuery

Relies on the Xpath (version 2.0) language

Can generate new XML documents

XQuery 1 and 2.2 does not define updating, see XQupdate (XML Query Updating)

Standards:

http://www.w3.org/TR/xquery/ (query, stable)

http://www.w3.org/TR/xqupdate/ (updates, recent)

Introductory examples

Find all nodes with path //topic/title

(returns all fragments <topic><title>xxxx </title></topic> from anywhere in the tree

for $t in //topic/title
   return $t 

Find all nodes of //topic/title in "catalog.xml"

for $t in document("catalog.xml")//topic/title
   return $t 

same, but via HTTP

for $t in document("http://tecfa.unige.ch/proj/seed/catalog/net/catalog.xml")//topic/title
   return $t 

Result

<title>TECFA Seed Catalog</title>
<title>Introduction</title>
<title>Conceptual and technical framework</title>
<title>The socio-constructivist  .... etc.

Use contexts

You need an XQuery processor

included in some command line/library products like “saxon”

included in XML databases.

included in an XML editor

included in some programming languages

Same principle as for SQL

From the command line

With an administration tool (Web or local)

Trough an other web application)

Basic XQuery

The query expression

Recall: XQuery uses XPath elements

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

//c3msbrick//c3mssoft[@id="epbl"]

functions

ex: only returns the content of a node

//c3msbrick/title/text()

FLWOR expressions

FLOWR = "For-Let-Where-Order-Return"

Similar to select-from-where-..-order-by of SQL

Overview:

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 with the XPath (see The query expression)

Example:

for $t in //topic/title return $t

let

"let" assigns a value (node) to a variable

Example without let:

for $t in document("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 document("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 document("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 document("catalog09.xml")//c3msbrick
  let $n := count($t//c3mssoft)
  where ($n > 1)
return <result> {$t/title/text()} owns {$n} bricks </result>

order

Can sort results

Alphabetic sorting:

for $t in //topic/title order by $t return $t

Alphabetic sorting

for $t in document("catalog09.xml")//c3msbrick
let $n := count($t//c3mssoft)
where ($n > 1)
order by $n
return <result> {$t/title/text()} owns {$n} bricks </result>

Slightly more complex return clause, we also include <titles> of <c3msbricks>:

for $t in document("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 document("catalog09.xml")//c3msbrick
let $n := count($t//c3mssoft)
return <result> 
       {$t/title/text()} owns {$n} bricks 
       </result>

Bad:

for $t in document("catalog09.xml")//c3msbrick
let $n := count($t//c3mssoft)
return $t/title/text() owns $n bricks 

Building XML fragments

Result of an xquery usually should lead to a single node (not a list)

Multi-fragment version (collection)

Consider the following expression

for $t in //c3msbrick/title
   return $t

It will return a list (a 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)

Version single fragment

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 expression within { ...} are evaluated

but all <tags> are copied as is ...

For within for

you may have loops within loops...

 <result>
 <title>List of C3MSBricks and associated Software</title>
 { for $t in document("catalog09.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>

Each FLWOR expression is 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 > Gallery owns les 5 bricks suivants:
    <soft > PhotoShare </ soft >
    <soft > Photoshare combined with PageSetter </ soft >
    <soft > My_eGallery </ soft >
    <soft > Coppermine </ soft >
    <soft > Gallery </ soft >
  </ brick >
 <brick > User statistics owns les 5 bricks suivants:
    <soft > commArt </ soft >
    <soft > pncUserPoints </ soft >
    <soft > pncSimpleStats </ soft >
    <soft > Statistics module </ soft >
    <soft > NS-User_Points </ soft >
 </ brick > 
 .....
 </result>

Links

Software and standards

See the XQuery article.

XQuery Tutorials

• XQuery (Wikipedia article)
• XQuery - Wikibook
• 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)

XQuery Web sites

• DataDirect's XQuery Developer Center (tutorials, examples, software)