oracle 删除not null非空约束

system@ORCL> create table testnull(id int not null,name varchar2(10) not null);
Table created.
system@ORCL> desc testnull
Name                                                                                Null?    Type
-------------------------------------------- -------- --------------------------------------------------------
ID                                           NOT NULL NUMBER(38)
NAME                                         NOT NULL VARCHAR2(10)
system@ORCL> desc DBA_CONS_COLUMNS
Name                                         Null?    Type
-------------------------------------------- -------- --------------------------------------------------------
OWNER                                        NOT NULL VARCHAR2(30)
CONSTRAINT_NAME                              NOT NULL VARCHAR2(30)
TABLE_NAME                                   NOT NULL VARCHAR2(30)
COLUMN_NAME                                                                                  VARCHAR2(4000)
POSITION                                                                                     NUMBER

system@ORCL> COL COLUMN_NAME FOR  A20
system@ORCL>  SELECT CONSTRAINT_NAME,COLUMN_NAME FROM DBA_CONS_COLUMNS WHERE TABLE_NAME='TESTNULL'
CONSTRAINT_NAME                COLUMN_NAME
------------------------------ --------------------
SYS_C0012465                   ID
SYS_C0012466                   NAME
system@ORCL> alter table testnull drop constraint SYS_C0012465;
Table altered.
system@ORCL> desc testnull;
Name                             Null?    Type
----------------------------- -------- --------------------------------------------------------
ID                                     NUMBER(38)
NAME                          NOT NULL VARCHAR2(10)
system@ORCL> alter table testnull name varchar2(10) null;
alter table testnull name varchar2(10) null
*
ERROR at line 1:
ORA-01735: invalid ALTER TABLE option
system@ORCL> alter table testnull modify name varchar2(10) null;
Table altered.
system@ORCL> desc testnull;
Name                              Null?    Type
--------------------------------- -------- --------------------------------------------------------
ID                                         NUMBER(38)
NAME                                       VARCHAR2(10)