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Specifying Boolean Expressions and Functions

This section provides information on how to specify Boolean expressions and functions in queries. It includes the following topics:

Using Boolean Expressions

You can specify Boolean expressions in the subqueries in filters. You specify the Boolean AND, OR, and NOT operators like this:

You can use parentheses to group collection specifications and operators for clarity or where the normal precedence is inadequate to express an operation.

Case Sensitivity

Operators are case sensitive. Spaces are not significant. You can omit them or include them for clarity.

Examples

The following query returns all authors who have at least one degree and one award:

author[degree and award] 
               

            

The next query finds all authors who have at least one degree or award and at least one publication:

author[(degree or award) and publication] 
               

            

Following is a query that finds all authors who have at least one degree and no publications:

author[degree and not(publication)] 
               

            

Calling Boolean Functions

This section describes the Boolean functions that you can call in a query. The operations you can perform are

Converting an Object to Boolean

In some situations, you might want to force a Boolean comparison. The XPath processor performs a Boolean comparison if either operand is a Boolean value. Consequently, if neither operand is a Boolean value, call the boolean() function on one operand to convert it to a Boolean value. The XPath processor automatically converts the other operand to a Boolean value. The format of the boolean() function is

boolean boolean(object) 
       

    

The boolean() function converts its argument to Boolean as follows:

The boolean() function is useful in comparisons. For example, the following query returns b elements that either contain both c and d elements as children or contain neither c nor d elements as children:

/a/b[boolean(c) = d]
               

            

This query is equivalent to the following query:

/a/b [(c and d) or (not(c) and not(d))]
               

            

Obtaining Boolean Values

To obtain the opposite Boolean value, call the not() function. The format is

boolean not(boolean) 
               

            

The not() function returns true if its argument is false, and returns false if its argument is true. For example, the following query finds all authors who have publications but no degrees or awards:

author[not(degree or award) and publication] 
               

            

To obtain the value true, call the true() function. The format is

boolean true() 
               

            

The true() function returns true.

To obtain the value false, call the false() function. The format is

boolean false() 
               

            

The false() function returns false.

Determining the Context Node Language

To determine whether the language of the context node is the language you expect it to be, call the lang() function. The format is

boolean lang(string) 
               

            

The lang() function returns true or false depending on whether the language of the context node as specified by the xml:lang attribute is the same as, or is a sublanguage of, the language specified by the argument string. The language of the context node is determined by the value of the xml:lang attribute on the context node or, if the context node has no xml:lang attribute, by the value of the xml:lang attribute on the nearest ancestor of the context node that has an xml:lang attribute.

If there is no such attribute, then lang() returns false. If there is such an attribute, lang() returns true in the following situations:

In both situations, case is ignored. For example:

lang("en")
       

    

This returns true if the context node is any of these elements: