Variables can optionally be declared (and bound) in the query prolog. If a variable is bound within an expression in the query body, it does not have to be declared in the prolog as well. For example, you can use the expression let $myInt := 2 in the query body without declaring $myInt in advance. $myInt is bound when the let expression is evaluated.

However, it is sometimes necessary to declare variables in the prolog, such as when:

Declaring variables in the prolog can also be a useful way to define constants, or values that can be calculated up front and used throughout the query. It's important to remember that global variables (prolog-declared variables) are immutable, just like other XQuery variables.

Variable Declaration Syntax

The syntax of a variable declaration is shown in Figure 12-4. For example, the declaration:

declare variable $maxItems := 12;

binds the value 12 to the variable $maxItems.

Figure 12-4. Syntax of a variable declaration^[a]

Important

A previous draft of the XQuery recommendation specified the following syntax for variable declarations:

 define variable $maxItems {12}

Some popular XQuery implementations still use this old syntax.