XQuery tutorial - basics: Difference between revisions

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Introduction

This is a beginners tutorial for XQuery.

Prerequisites:

Basic knowledge of XML
Some XPath

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:

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()

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 the tree

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/catalog.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

XQuery Web sites

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

@@ Line 298: / Line 298: @@
 === XQuery Tutorials ===
+See also the XQuery article
 * [http://en.wikipedia.org/wiki/XQuery XQuery] (Wikipedia article)
@@ Line 310: / Line 312: @@
 * Michael Brundage (2004). XQuery: The XML Query Language, Portland: Book News [this is a good book]
 * [http://www.idealliance.org/proceedings/xtech05/papers/02-03-01/ Comparing XSLT and XQuery] by Michael Kay, Saxonica (not really a tutorial, but useful)
+* [http://www.stylusstudio.com/xquery_primer.html Learn XQuery in 10 Minutes: An XQuery Tutorial] and [http://www.stylusstudio.com/xquery_flwor.html Blooming FLWOR - An Introduction to the XQuery FLWOR Expression] by Michael Kay (but centered on a commercial editor)
 === XQuery Web sites ===

XQuery tutorial - basics: Difference between revisions

Revision as of 19:12, 11 February 2010

Contents

Introduction

Use contexts

Basic XQuery

FLWOR expressions

Some XQuery tricks

About creating XML fragments

For within for

Links

Software and standards

XQuery Tutorials

XQuery Web sites

Navigation menu

XQuery tutorial - basics: Difference between revisions

Revision as of 19:12, 11 February 2010

Introduction

Use contexts

Basic XQuery

FLWOR expressions

Some XQuery tricks

About creating XML fragments

For within for

Links

Software and standards

XQuery Tutorials

XQuery Web sites

Navigation menu

Slow Search