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XQuery Functions

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XQuery 1.0, XPath 2.0, and XSLT 2.0 share the same functions library.

XQuery Functions

XQuery includes over 100 built-in functions. There are functions for string values, numeric values, date and time comparison, node and QName manipulation, sequence manipulation, Boolean values, and more. You can also define your own functions in XQuery.

XQuery Built-in Functions

The URI of the XQuery function namespace is:

The default prefix for the function namespace is fn:.

Tip: Functions are often called with the fn: prefix, such as fn:string(). However, since fn: is the default prefix of the namespace, the function names do not need to be prefixed when called.

The reference of all the built-in XQuery 1.0 functions is located in our XPath tutorial.

Examples of Function Calls

A call to a function can appear where an expression may appear. Look at the examples below:

Example 1: In an element


Example 2: In the predicate of a path expression


Example 3: In a let clause

let $name := (substring($booktitle,1,4))

XQuery User-Defined Functions

If you cannot find the XQuery function you need, you can write your own.

User-defined functions can be defined in the query or in a separate library.


declare function prefix:function_name($parameter AS datatype)
  AS returnDatatype
(: ...function code here... :)

Notes on user-defined functions:

  • Use the declare function keyword
  • The name of the function must be prefixed
  • The data type of the parameters are mostly the same as the data types defined in XML Schema
  • The body of the function must be surrounded by curly braces

Example of a User-defined Function Declared in the Query

declare function local:minPrice(
  $price as xs:decimal?,
  $discount as xs:decimal?)
  AS xs:decimal?
let $disc := ($price * $discount) div 100
return ($price - $disc)
(: Below is an example of how to call the function above :)
<minPrice>{local:minPrice($book/price, $book/discount)}</minPrice>

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