Calc Object Function isnull() - test a field or variable for SQL NULL
 
   Format:
     isnull(<field-variable-name>,<alternate-value))
       <field-variable-name> is a string or numeric field or variable
 
       <alternate-value> is a string or numeric constant of the same type
         as <field-variable-name>
 
   Result:
     The result is the same data type as <field-variable-name>