boolean function

The fn:boolean function returns the effective Boolean value of a sequence.

Syntax

sequence-expression: Any sequence that contains items of any type, or the empty sequence.

The returned effective Boolean value (EBV) depends on the value of sequence-expression:

Table 1. EBVs returned for specific types of values in XQuery
Description of value	EBV returned
An empty sequence	false
A sequence whose first item is a node	true
A single value of type xs:boolean (or derived from xs:boolean)	false - if the xs:boolean value is false
	true - if the xs:boolean value is true
A single value of type xs:string or xdt:untypedAtomic (or derived from one of these types)	false - if the length of the value is zero
	true - if the length if the value is greater than zero
A single value of any numeric type (or derived from a numeric type)	false - if the value is NaN or is numerically equal to zero
	true - if the value is not numerically equal to zero
All other values	error
Note: The effective Boolean value of a sequence that contains at least one node and at least one atomic value is nondeterministic in a query where the order is unpredictable.

Example with an argument that is a single numeric value: The following function returns the effective Boolean value of 0:

fn:boolean(0)

The returned value is false.

Example with an argument that is a multiple-item sequence: The following function returns the effective Boolean value of (<a/>, 0, <b/>):

fn:boolean((<a/>, 0, <b/>))

The returned value is true.