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 | + | 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 lang='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 lang='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 == | ||
Returns dynamically named elements: | Returns dynamically named elements: | ||
− | <pre | + | <pre lang='xquery'> |
− | let $ | + | let $root := "element" |
− | let $contents :=<foo | + | let $value := "hi" |
− | return element {$ | + | let $contents := <foo>Bar!</foo> |
− | + | return element { $root } { | |
+ | attribute { "about" } { $value }, $contents | ||
} | } | ||
</pre> | </pre> | ||
− | [[ | + | |
− | + | The result is an XML fragment with <code><element></code> as root node: | |
+ | <pre lang="xml"> | ||
+ | <element about="hi"> | ||
+ | <foo>Bar!</foo> | ||
+ | </element> | ||
+ | </pre> | ||
+ | |||
+ | == Transform List to Tree == | ||
+ | This snippet transform a ''flat'' list of elements with <code>parentId</code>-references to a nested list. | ||
+ | |||
+ | <pre lang='xquery'> | ||
+ | declare function local:link($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') | ||
+ | </pre> | ||
+ | results in | ||
+ | <pre 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> | ||
+ | </pre> | ||
+ | |||
+ | == IP-Converter == | ||
+ | |||
+ | This snippet converts an IP address to its numeric representation: | ||
+ | |||
+ | <pre lang='xquery'> | ||
+ | let $ip := '134.34.226.65' | ||
+ | return fold-left( | ||
+ | tokenize($ip, '\.')!xs:integer(.), | ||
+ | 0, | ||
+ | function($n, $d) { 256 * $n + $d } | ||
+ | ) | ||
+ | </pre> | ||
+ | results in | ||
+ | <pre lang="xml"> | ||
+ | 2250433089 | ||
+ | </pre> | ||
+ | |||
+ | == Count number of files == | ||
+ | |||
+ | This snippets returns the number of JPG files in a directory and its subdirectories: | ||
+ | |||
+ | <pre lang='xquery'> | ||
+ | basex "count(file:list('.',true(),'*.jpg'))"</pre> | ||
+ | |||
+ | The Linux equivalent is | ||
+ | |||
+ | <pre lang="xml"> | ||
+ | find . | grep \.jpg$ | wc -l | ||
+ | </pre> |
Latest revision as of 18:38, 1 December 2023
This page contains code snippets that mainly originate from our basex-talk mailing list.
Contents
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 }, $contents
}
The result is an XML fragment with <element>
as root node:
<element about="hi">
<foo>Bar!</foo>
</element>
Transform List to Tree[edit]
This snippet transform a flat list of elements with parentId
-references to a nested list.
declare function local:link($entries as node()*, $id as xs:string) {
let $entry := $entries[@id eq $id],
$children := $entries[@parentId eq $id]
return element entry {
$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[edit]
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 + $d }
)
results in
2250433089
Count number of files[edit]
This snippets returns the number of JPG files in a directory and its subdirectories:
basex "count(file:list('.',true(),'*.jpg'))"
The Linux equivalent is
find . | grep \.jpg$ | wc -l