UNIQUE

    - It doesn't accept duplicate values.
    - It allows NULL values.

    Syntax:
        column_name datatype [size] CONSTRAINT <constraint_name> unique;
   
    Examples:
   
    (i) Creating a UNIQUE constraint.

        Column Level Unique Declaration.
            create table emp
            ( empno number(4), ename varchar2(20) CONSTRAINT unique_ename UNIQUE ename, ......);
       
        Table Level Unique Declaration.
            create table emp
            (empno number(4), ename varchar2(20), ..... ,
                CONSTRAINT unique_ename UNIQUE ename );
           
        Using Alter
            create table emp
            (empno number(4), ename varchar2(20), ..... );

            Alter table emp add ( CONSTRAINT unique_ename UNIQUE ename );
       
    (ii) Dropping a UNIQUE constraint.

        To drop a UNIQUE constraint, we need to know the name used for the constraint.

        Alter table emp drop constraint unique_ename;