SQL> desc sunil
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ID                                        NOT NULL NUMBER(2)
 FIRST_NAME                                NOT NULL VARCHAR2(20)
 LAST_NAME                                 NOT NULL VARCHAR2(20)
 DOB                                                DATE
 SEX                                                VARCHAR2(1)
 QUALIFICATION                                      VARCHAR2(20)
 AGE                                                NUMBER(2)



SQL> alter table sunil
  2  add constraint sexnn not null (sex);
add constraint sexnn not null (sex)
                     *
ERROR at line 2:
ORA-00904: invalid column name


SQL> alter table sunil
  2  add constraint not null (sex);
add constraint not null (sex)
                         *
ERROR at line 2:
ORA-01735: invalid ALTER TABLE option