Lets you store the missing value for a field with the STORE
    statement or the MODIFY statement. When a field is referred to
    as missing, you can think of the field as empty, a field in which
    no value is stored.

    In order to use RDB$MISSING, you must have previously defined
    a missing value for the field when you defined the database.
    If a field is left blank, or you use RDB$MISSING without having
    defined a missing value for that field in its field definition,
    RDML returns an error.