How to add unique key constraint to an existing column of a table?

Here below is a sample SQL statement to add unique key constraint to an existing column of a table:

SQL> conn mereba/mereba@to_primary
Connected.

--- Add unique constraint to column 'EMP_ID'  of table emp_sal
SQL> alter table emp_sal add constraint uk_impID UNIQUE (EMP_ID);
alter table emp_sal add constraint uk_impID UNIQUE (EMP_ID)
                                            *
ERROR at line 1:
ORA-02261: such unique or primary key already exists in the table

---NOTE: on oracle database, primary column of a table also becomes unique column of the table by default.

----Create unique constraint on non-primary column of the table
SQL> alter table emp_sal add constraint uk_jobtitle UNIQUE (jobtitle);
 Table altered.
SQL>