A Boolean expression is the logical representation of a
       relationship between value expressions. The value of a Boolean
       expression is either true or false. When dealing with relational
       sources, the value of a Boolean expression is either TRUE, FALSE,
       or UNKNOWN according to SQL/Services standards. The UNKNOWN
       boolean value may result if either value expression in a Boolean
       expression is null.

       You can use Boolean expressions in the following DEC DATATRIEVE
       clauses and statements:

       o  WITH clause in a record selection expression

       o  WITH clause in a SELECT statement

       o  IF clause of an IF-THEN-ELSE statement

       o  IF clause of an IF-THEN-ELSE value expression

       o  CHOICE statement

       o  CHOICE value expression

       o  VALID IF clause in a record definition

       o  WHILE statement

       The DEC DATATRIEVE CHOICE and IF-THEN-ELSE value expressions, and
       the CHOICE and IF-THEN-ELSE statements can contain an additional
       ELSE clause to support the UNKNOWN boolean value.

       Boolean expressions consist of value expressions, relational
       operators, and Boolean operators. Relational operators control
       the comparison of value expressions. Boolean operators enable you
       to join two or more Boolean expressions and to reverse the value
       of a Boolean expression. All Boolean expressions contain value
       expressions and relational operators, and some contain Boolean
       operators.