XQuery Recipes

From BaseX Documentation
Jump to navigation Jump to search

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

Compact Notations[edit]

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

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:
let $s := "X" return (
  (: OLD :) if(exists($s)) then $s else 'default',
  (: NEW :) ($s, 'default')[1]

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

Computed Elements[edit]

Returns dynamically named elements:

let $root := "element"
let $value := "hi"
let $contents := <foo>Bar!</foo>
return element { $root } {
  attribute { "about" } { $value }, $contents

The result is an XML fragment with <element> as root node:

<element about="hi">

Transform List to Tree[edit]

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

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 {
   for $child in $children
   return local:link($entries, $child/@id)

let $entries :=
	  <entry id="entry1" />
	  <entry id="entry2" parentId="entry1" />
	  <entry id="entry3" parentId="entry1" />
	  <entry id="entry4" parentId="entry2" />
	  <entry id="entry5" parentId="entry2" />
	  <entry id="entry6" parentId="entry3" />
	  <entry id="entry7" parentId="entry3" />
return local:link($entries/entry, 'entry1')

results in

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


This snippet converts an IP address to its numeric representation:

let $ip := ''
return fold-left(
   tokenize($ip, '\.')!xs:integer(.),
  function($n, $d) { 256 * $n + $d }

results in


Count number of files[edit]

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

basex "count(file:list('.',true(),'*.jpg'))"

The Linux equivalent is

find . | grep \.jpg$ | wc -l