XQuery tutorial - basics

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Introduction

This is a beginners tutorial for XQuery.

Prerequisites:

Recommended reading, before this:

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.

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 documents
XQuery does not define updating, see XQupdat (as of Jan 2010 still not a full recommendation, but widely implemented)

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.
  • 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:

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 example shows how to find all nodes of //topic/title in a file called "catalog.xml": 

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

The next example does the same, but retrieves the file via HTTP: 

for $t in document("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.

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 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

Example of an alphabetic sorting: 

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

More complex alphabetic sorting example: 

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>

Below is a 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

Some XQuery tricks

About creating XML fragments

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 expression within { ...} are evaluated
but all tags are copied as is ...

For within for

You may have loops within loops as the following example shows: 

 <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.

According to the W3C, there are over 40 different software packages that support XML Query in some way.

XQuery Tutorials

See also the XQuery article

XQuery Web sites

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