A value expression is a symbol or string of symbols that you
    use to calculate a value. When you use a value expression in
    a statement, Oracle Rdb calculates the value associated with the
    expression and uses it when executing the statement.

    The RDML value expressions are:

    o  Arithmetic

    o  Database Field Value

    o  FIRST FROM

    o  Host Variable

    o  RDB$DB_KEY

    o  RDB$MISSING

    For more information and the syntax diagrams see the HELP entries
    for each of these expressions.