Effective Boolean Value (XQuery)

Applies to: SQL Server

These are the effective Boolean values:

• False if the operand is an empty sequence or a Boolean false.

• Otherwise, the value is true.

The effective Boolean value can be computed for expressions that return a single Boolean value, a node sequence, or an empty sequence. Note that the Boolean value is computed implicitly when the following types of expressions are processed:

• Logical expressions

• The not function

• The WHERE clause of a FLWOR expression

• Conditional expressions

• QuantifiedeExpressions

Following is an example of an effective Boolean value. When the if expression is processed, the effective Boolean value of the condition is determined. Because /a[1] returns an empty sequence, the effective Boolean value is false. The result is returned as XML with one text node (false).

value is false  
DECLARE @x XML  
SET @x = '<b/>'  
SELECT @x.query('if (/a[1]) then "true" else "false"')  
go  

In the following example, the effective Boolean value is true, because the expression returns a nonempty sequence.

DECLARE @x XML  
SET @x = '<a/>'  
SELECT @x.query('if (/a[1]) then "true" else "false"')  
go  

When querying typed xml columns or variables, you can have nodes of Boolean type. The data() in this case returns a Boolean value. If the query expression returns a Boolean true value, the effective Boolean value is true, as shown in the next example. The following is also illustrated in the example:

• An XML schema collection is created. The element <b> in the collection is of Boolean type.

• A typed xml variable is created and queried.

• The expression data(/b[1]) returns a Boolean true value. Therefore, the effective Boolean value in this case is true.

• The expression data(/b[2]) returns a Boolean false value. Therefore, the effective Boolean value in this case is false.

CREATE XML SCHEMA COLLECTION SC AS '  
<schema xmlns="http://www.w3.org/2001/XMLSchema">  
      <element name="s" type="string"/>  
      <element name="b" type="boolean"/>  
</schema>'  
go  
DECLARE @x XML(SC)  
SET @x = '<b>true</b><b>false</b>'  
SELECT @x.query('if (data(/b[1])) then "true" else "false"')  
SELECT @x.query('if (data(/b[2])) then "true" else "false"')  
go  

See Also

XQuery Basics
FLWOR Statement and Iteration (XQuery)