NULL_PERM = {Y | N | D} [, {Y | N | D}] ...;
where the specification is listed in the same order as the corresponding field names, and delimited by commas.
If NULL_PERM is set to Y, NULL values are permitted in the column. If NULL_PERM is set to N, the column is prevented from containing NULL values. If NULL_PERM is set to D, NULL values are converted to a specified default value for the column.