NULL predicate

The NULL predicate tests for null values.

Read syntax diagram
>>-expression--IS--+-----+--NULL-------------------------------><
                   '-NOT-'         

The result of a NULL predicate cannot be unknown. If the value of the expression is null, the result is true. If the value is not null, the result is false. If NOT is specified, the result is reversed.

A parameter marker must not be specified for or within the expression.

Example 1: The following predicate is true whenever PHONENO has the null value, and is false otherwise. 
  PHONENO IS NULL