XQuery 1.0, XPath 2.0, and XSLT 2.0 share the same functions library.
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.
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.
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
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.
Notes on user-defined functions: