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/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

Introductory examples

'Find all nodes with path //topic/title'

The following expression 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

Let's recall that XQuery uses XPath elements. Below are some simple examples:

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.

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

XQuery Tutorials

XQuery Web sites

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