SQL & PL/SQL

Hi,

I've many tables with columns "DEFAULT NULL NOT NULL".
These are my create table scripts

CREATE TABLE TAB_1
(
  COL1       VARCHAR2(15 BYTE)                DEFAULT NULL                  NOT NULL,
  COL2       VARCHAR2(255 BYTE)               DEFAULT NULL                  NOT NULL,
 ID  INTEGER                          NOT NULL,
  COL3  VARCHAR2(255 BYTE)               DEFAULT NULL
);
ALTER TABLE TAB_1 ADD (CONSTRAINT TAB_1_PK PRIMARY KEY (ID);



CREATE TABLE TAB_2
(
  ID1       VARCHAR2(15 BYTE)                DEFAULT NULL                  NOT NULL,
  ID2       VARCHAR2(255 BYTE)               DEFAULT NULL                  NOT NULL,
  COL4  INTEGER                          NOT NULL,
  COL5  VARCHAR2(255 BYTE)               DEFAULT NULL
);
ALTER TABLE TAB_2 ADD (CONSTRAINT TAB_2_PK PRIMARY KEY (ID1, ID2);

I'd like to modify my columns in (for example):

 COL1       VARCHAR2(15 BYTE)    NOT NULL,   
 OR
 COL1       VARCHAR2(15 BYTE),

I tried:

ALTER TABLE TAB_1 MODIFY (COL1       VARCHAR2(15 BYTE));

But I get always:

 COL1       VARCHAR2(15 BYTE)                DEFAULT NULL                  NOT NULL

Have you any idea?