XQuery Recipes

This page contains code snippets that mainly originate from our basex-talk mailing list.

Compact Notations

if/not/else constructs can look pretty verbose in XQuery. However, some alternatives exist in order to make conditional code more compact:

• The Simple Map Operator can be used to trigger an action if a value has a single item. The following two expressions are equivalent:

<syntaxhighlight lang="xquery"> let $s := "X" return (

 (: OLD :) if(count($s) = 1) then 'OK' else (),
 (: NEW :) $s ! 'OK'

) </syntaxhighlight>

In some cases, also the first solution can be written more compact. If we know that our input will always have 0-1 items, we can write if(exists($s)). If our input will never be an empty string, a zero, etc, it’s sufficient to write if($s).

• If you want to choose the first non-empty item from two arguments, we can use the position predicate:

<syntaxhighlight lang="xquery"> let $s := "X" return (

 (: OLD :) if(exists($s)) then $s else 'default',
 (: NEW :) ($s, 'default')[1]

) </syntaxhighlight>

Note that this only works if both of your inputs have zero or one items.

Computed Elements

Returns dynamically named elements: <syntaxhighlight lang="xquery"> let $root := "element" let $value := "hi" let $contents := <foo>Bar!</foo> return element { $root } {

 attribute { "about" } { $value }, $contents

} </syntaxhighlight>

The result is an XML fragment with <element> as root node: <syntaxhighlight lang="xml"> <element about="hi">

 <foo>Bar!</foo>

</element> </syntaxhighlight>

Transform List to Tree

This snippet transform a flat list of elements with parentId-references to a nested list.

<syntaxhighlight lang="xquery"> declare function local:link($entries as node()*, $id as xs:string) {

let $entry    := $entries[@id eq $id],
    $children := $entries[@parentId eq $id]
return element entry {
  $entry/@*,
  for $child in $children
  return local:link($entries, $child/@id)
}

};

let $entries :=

<entries>

<entry id="entry1" /> <entry id="entry2" parentId="entry1" /> <entry id="entry3" parentId="entry1" /> <entry id="entry4" parentId="entry2" /> <entry id="entry5" parentId="entry2" /> <entry id="entry6" parentId="entry3" /> <entry id="entry7" parentId="entry3" />

</entries>

return local:link($entries/entry, 'entry1') </syntaxhighlight> results in <syntaxhighlight lang="xml"> <entry id="entry1">

 <entry id="entry2" parentId="entry1">
   <entry id="entry4" parentId="entry2"/>
   <entry id="entry5" parentId="entry2"/>
 </entry>
 <entry id="entry3" parentId="entry1">
   <entry id="entry6" parentId="entry3"/>
   <entry id="entry7" parentId="entry3"/>
 </entry>

</entry> </syntaxhighlight>

IP-Converter

This snippet converts an IP address to its numeric representation:

<syntaxhighlight lang="xquery"> let $ip := '134.34.226.65' return fold-left(

  tokenize($ip, '\.')!xs:integer(.),
  0,
 function($n, $d) { 256 * $n + $d }

) </syntaxhighlight> results in <syntaxhighlight lang="xml"> 2250433089 </syntaxhighlight>

Count number of files

This snippets returns the number of JPG files in a directory and its subdirectories:

<syntaxhighlight lang="xquery"> basex "count(file:list('.',true(),'*.jpg'))"</syntaxhighlight>

The Linux equivalent is

<syntaxhighlight lang="xml"> find . | grep \.jpg$ | wc -l </syntaxhighlight>