o  The default value for a field ("Rdb default" in displays by the
       SQL statement SHOW TABLE table-name) is not the same as the missing
       value that you can specify with RDO.  If you do not specify a value
       for a field (column) with a default value, the default value is
       actually stored in the database. This is true whether you are using
       RDO or SQL.

       An RDO missing value is not actually stored in the database. If you
       use RDO to specify a missing value for a field, that missing value
       is displayed by RDO when the field has no value stored and the
       internal null flag is set. SQL does not recognize any missing value
       specified by RDO; if the field has no value stored and the null flag
       is set, then SQL displays "NULL" for the column, regardless of
       whether you specified any missing value with RDO.

       One implication of the way in which Oracle Rdb handles default values
       is that if you change the default value for a column, it has no
       effect on any existing data in the database; that is, rows stored
       with columns containing the "old" default values are not changed.
       By contrast, changing the missing value does change what is displayed
       by RDO-based applications for columns that have no value stored and
       that have the null flag set.

    o  The value of RDB$MISSING is set at compile time.  If the missing
       value for a field is changed, then sources that contain references
       to RDB$MISSING for that field still use the old missing value.  To
       correct this problem, all sources that contain a reference to
       RDB$MISSING on the field must be recompiled.