Represents the relationship between two value expressions.
    Sometimes called a Boolean expression, conditional expressions
    are used in the WITH clause of the record selection expression.

    The value of a conditional expression is true, false, or missing.

    The two types of conditional expressions are:

    o  Those that express a relationship between two value
       expressions, using a relational operator

       For example, the expression SH.SALARY_AMOUNT > 50000 is true
       if the value in the SALARY_AMOUNT field of the SALARY_HISTORY
       record is greater than 50000. When Oracle Rdb evaluates this
       expression, it examines the relationship between the two value
       expressions, SH.SALARY_AMOUNT and 50000.

    o  Those that express a characteristic of a single value
       expression

       For example, E.EMPLOYEE_ID MISSING is true if there is no
       value in the EMPLOYEE_ID field of an EMPLOYEES record.

    The RDML conditional expressions are:

    -  ANY

    -  BETWEEN

    -  CONTAINING

    -  MATCHING

    -  MISSING

    -  Relational Operators

    -  STARTING WITH

    -  UNIQUE

    For more information and the syntax diagram of each of these
    expressions, see the individual HELP entries.