Difference between revisions of "XQuery Recipes"
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This page contains code snippets that mainly originate from our [https://mailman.uni-konstanz.de/mailman/listinfo/basex-talk basex-talk] mailing list. | This page contains code snippets that mainly originate from our [https://mailman.uni-konstanz.de/mailman/listinfo/basex-talk basex-talk] mailing list. | ||
+ | |||
+ | == Compact Notations == | ||
+ | |||
+ | <code>if</code>/<code>not</code>/<code>else</code> constructs can look pretty verbose in XQuery. | ||
+ | However, some alternatives exist in order to make conditional code more compact: | ||
+ | |||
+ | * The [[XQuery 3.0#Simple Map Operator|Simple Map Operator]] can be used to trigger an action if a value has a single item. The following two expressions are equivalent: | ||
+ | |||
+ | <pre class="brush:xquery"> | ||
+ | let $s := "X" return ( | ||
+ | (: OLD :) if(count($s) = 1) then 'OK' else (), | ||
+ | (: NEW :) $s ! 'OK' | ||
+ | ) | ||
+ | </pre> | ||
+ | |||
+ | In some cases, also the first solution can be written more compact. If we know that our input will always have 0-1 items, we can write <code>if(exists($s))</code>. If our input will never be an empty string, a zero, etc, it’s sufficient to write <code>if($s)</code>. | ||
+ | |||
+ | * If you want to choose the first non-empty item from two arguments, we can use the position predicate: | ||
+ | |||
+ | <pre class="brush:xquery"> | ||
+ | let $s := "X" return ( | ||
+ | (: OLD :) if(exists($s)) then $s else 'default', | ||
+ | (: NEW :) ($s, 'default')[1] | ||
+ | ) | ||
+ | </pre> | ||
+ | |||
+ | Note that this only works if both of your inputs have zero or one items. | ||
== Computed Elements == | == Computed Elements == | ||
Line 66: | Line 93: | ||
let $ip := '134.34.226.65' | let $ip := '134.34.226.65' | ||
return fold-left( | return fold-left( | ||
− | + | tokenize($ip, '\.')!xs:integer(.), | |
− | + | 0, | |
− | + | function($n, $d) { 256 * $n + $d } | |
) | ) | ||
</pre> | </pre> | ||
Line 88: | Line 115: | ||
find . | grep \.jpg$ | wc -l | find . | grep \.jpg$ | wc -l | ||
</pre> | </pre> | ||
− | |||
− |
Revision as of 23:41, 8 July 2017
This page contains code snippets that mainly originate from our basex-talk mailing list.
Contents
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:
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
Returns dynamically named elements:
let $root := "element" let $value := "hi" let $contents := <foo>Bar!</foo> return element { $root } { attribute { "about" } { $value }, $contents }
The result is an XML fragment with <element>
as root node:
<element about="hi"> <foo>Bar!</foo> </element>
Transform List to Tree
This snippet transform a flat list of elements with parentId
-references to a nested list.
declare function local:link($entries as node()*, $id as xs:string) { let $entry := $entries[@id eq $id], $children := $entries[@parentId eq $id] return element entry { $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')
results in
<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>
IP-Converter
This snippet converts an IP address to its numeric representation:
let $ip := '134.34.226.65' return fold-left( tokenize($ip, '\.')!xs:integer(.), 0, function($n, $d) { 256 * $n + $d } )
results in
2250433089
Count number of files
This snippets returns the number of JPG files in a directory and its sub-directories:
basex "count(file:list('.',true(),'*.jpg'))"
The Linux equivalent is
find . | grep \.jpg$ | wc -l