DUP_VAL_ON_INDEX Exception

CREATE TABLE EMP (EMPNO NUMBER(4) NOT NULL,
                  ENAME VARCHAR2(10),
                  JOB VARCHAR2(9),
                  SAL NUMBER(7, 2),
                  DEPTNO NUMBER(2),
                  CONSTRAINT emp_pk PRIMARY KEY (empno)
);

INSERT INTO EMP VALUES (1, 'SMITH', 'CLERK',     800,    20);

SQL> BEGIN
  2      INSERT INTO emp (empno, ename) VALUES (1, 'Greg');
  3  EXCEPTION
  4  WHEN DUP_VAL_ON_INDEX THEN
  5   DBMS_OUTPUT.PUT_LINE('Duplicate value on an index');
  6  END;
  7  /
Duplicate value on an index

PL/SQL procedure successfully completed.

SQL>