creating unique key and primary key on same column is possible? already unique key and not null constrains there on same column but still i want to create primary key without dropping unique key.
That depends ... If you currently have a unique INDEX then you can add a primary key and it will automatically use that index
SQL> create table t ( x int );
Table created.
SQL> insert into t values (1);
1 row created.
SQL> commit;
Commit complete.
SQL>
SQL> create unique index ix on t ( x );
Index created.
SQL>
SQL> alter table t add constraint pk primary key ( x );
Table altered.
SQL>
SQL> select count(*) from user_indexes where table_name = 'T';
COUNT(*)
----------
1
However if you already have a unique CONSTRAINT then you cannot add a primary key on the same column
SQL> create table t ( x int );
Table created.
SQL> insert into t values (1);
1 row created.
SQL> commit;
Commit complete.
SQL>
SQL> alter table t add constraint ix unique ( x );
Table altered.
SQL>
SQL> alter table t add constraint pk primary key ( x );
alter table t add constraint pk primary key ( x )
*
ERROR at line 1:
ORA-02261: such unique or primary key already exists in the table
In that instance, making the column non nullable will be equivalent to a primary key