Not Null constraint

Hi,

I have two databases 9.2.0.5 (AIX)
with exactly the same table dvordtm

• on the first DB, CTAS give me NOT NULL constraint
• on ths second, CTAS does'nt give NOT NULL constraint

Do you know why ?
Can you help me ?
Thanks in advance ....

SQL> create table toto
  2 as select dtdate from dvordtm where 1=0;

SQL> desc toto

 Nom                                       NULL ?   Type
 ----------------------------------------- -------- -------------------

 DTDATE                                    NOT NULL DATE

SQL> create table toto
  2 as select dtdate from dvordtm where 1=0;

SQL> desc toto

 Nom                                       NULL ?   Type
 ----------------------------------------- -------- ---------------------

 DTDATE                                             DATE


SQL> desc dvordtm
 Nom                                       NULL ?   Type
 ----------------------------------------- -------- ---------------------

 DTDATE                                    NOT NULL DATE
 PERCPT                                             VARCHAR2(6)
 ACNTYY                                             VARCHAR2(4)
 ACNTMM                                             VARCHAR2(2)
 CDCLOSEDAY                                NOT NULL VARCHAR2(1)
 FLAG_LAST_CLOSED_PERIOD                   NOT NULL VARCHAR2(1)
 FLAG_LAST_CLOSED_YEAR                     NOT NULL VARCHAR2(1)
 FLAG_ACTUAL_PERIOD                        NOT NULL VARCHAR2(1)
 FLAG_ACTUAL_YEAR                          NOT NULL VARCHAR2(1)
 FLAG_PREVIOUS_CLOSED_PERIOD               NOT NULL VARCHAR2(1)
 FLAG_PREVIOUS_CLOSED_YEAR                 NOT NULL VARCHAR2(1)
 FLAG_PREVIOUS_PERIOD                      NOT NULL VARCHAR2(1)
 FLAG_PREVIOUS_YEAR                        NOT NULL VARCHAR2(1)
 CALENDAR_YEAR                             NOT NULL VARCHAR2(4)
 CALENDAR_WEEK                             NOT NULL VARCHAR2(2)