oracle字段 not null不一致

SQL> create table test(a number, b number);
 
Table created
 
SQL> desc test;
Name Type   Nullable Default Comments 
---- ------ -------- ------- -------- 
A    NUMBER Y                         
B    NUMBER Y                         
 
SQL> alter table test modify a not null novalidate;
 
Table altered
 
SQL> desc test;
Name Type   Nullable Default Comments 
---- ------ -------- ------- -------- 
A    NUMBER Y                         
B    NUMBER Y                         
 
SQL> insert into test(b) values(1);
 
insert into test(b) values(1)
 
ORA-01400: 无法将 NULL 插入 ("xx"."TEST"."A")

SQL> select c.constraint_name,c.constraint_type,cc.column_name from user_constraints c, user_cons_columns cc
  2  where c.table_name='TEST' and c.constraint_name=cc.constraint_name;
 
CONSTRAINT_NAME                CONSTRAINT_TYPE COLUMN_NAME
------------------------------ --------------- -----------------------------
SYS_C0051838                   C               A
 
SQL> alter table test modify constraint SYS_C0051838 enable validate;
 
Table altered
 
SQL> desc test;
Name Type   Nullable Default Comments 
---- ------ -------- ------- -------- 
A    NUMBER                           
B    NUMBER Y