SQL> create table Stu_Table(Stu_Id varchar(2), Stu_Name varchar(10),
  2  Stu_Class  varchar(10));

Table created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(1,'Komal');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(2,'Ajay');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(3,'Rakesh');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(4,'Bhanu');

1 row created.

SQL> insert into Stu_Table (Stu_Id, Stu_Name) values(5,'Santosh');

1 row created.

SQL> select * from Stu_Table;

ST STU_NAME   STU_CLASS
-- ---------- ----------
1  Komal
2  Ajay
3  Rakesh
4  Bhanu
5  Santosh

SQL> Describe Stu_Table
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------

 STU_ID                                             VARCHAR2(2)
 STU_NAME                                           VARCHAR2(10)
 STU_CLASS                                          VARCHAR2(10)

SQL>ALTER TABLE Stu_Table MODIFY Stu_Id int(3)not null;
ALTER TABLE Stu_Table MODIFY Stu_Id int(3)not null
                                       *
ERROR at line 1:
ORA-01735: invalid ALTER TABLE option

SQL> ALTER TABLE Stu_Table add C1_TEMP integer NOT NULL;
ALTER TABLE Stu_Table add C1_TEMP integer NOT NULL
            *
ERROR at line 1:
ORA-01758: table must be empty to add mandatory (NOT NULL) column

alter table stu_table modify stu_id not null;

SQL> select count(*) from t;
  COUNT(*)
----------
         4

1 row selected.

SQL> alter table t add (newcol varchar2(20) default 'X' not null);

Table altered.