Oracle 11g Release 2

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CREATE TABLE EMP(EMPNO INT,SAL NUMBER(8,2))
INSERT INTO EMP 
SELECT 7839, 4000 FROM dual UNION ALL SELECT 7840, 4500 FROM dual
2 rows affected

CREATE OR REPLACE TRIGGER PRACTICE_TRIGGER
BEFORE DELETE OR UPDATE OF SAL ON EMP
FOR EACH ROW
BEGIN
 IF :OLD.EMPNO = 7839 AND :OLD.SAL != NVL(:NEW.SAL,0) THEN
   RAISE_APPLICATION_ERROR('-20000','CAN''T UPDATE SALARY OR DELETE THE ROW FOR EMPNO = '||:OLD.EMPNO||' | :OLD.SAL = '||:OLD.SAL||' | :NEW.SAL = '||:NEW.SAL);
 END IF;
END;
/
UPDATE EMP SET SAL=4600 WHERE EMPNO=7840;
1 rows affected

UPDATE EMP SET SAL=4200 WHERE EMPNO=7839;
ORA-20000: CAN'T UPDATE SALARY OR DELETE THE ROW FOR EMPNO = 7839 | :OLD.SAL = 4000 | :NEW.SAL = 4200
ORA-06512: at "FIDDLE_FYUBTBPTWXLENBQPHRTJ.PRACTICE_TRIGGER", line 3
ORA-04088: error during execution of trigger 'FIDDLE_FYUBTBPTWXLENBQPHRTJ.PRACTICE_TRIGGER'

DELETE EMP WHERE EMPNO=7840;
1 rows affected

-- :NEW.EMPNO returns NULL
DELETE EMP WHERE EMPNO=7839; 
ORA-20000: CAN'T UPDATE SALARY OR DELETE THE ROW FOR EMPNO = 7839 | :OLD.SAL = 4000 | :NEW.SAL =
ORA-06512: at "FIDDLE_FYUBTBPTWXLENBQPHRTJ.PRACTICE_TRIGGER", line 3
ORA-04088: error during execution of trigger 'FIDDLE_FYUBTBPTWXLENBQPHRTJ.PRACTICE_TRIGGER'