Bad bind variable error

[yasir saleem]

CREATE OR REPLACE TRIGGER display_salary_changes
BEFORE UPDATE ON customers
FOR EACH ROW
WHEN (NEW.ID > 0)
DECLARE
sal_diff number;
BEGIN
sal_diff:=:NEW.SALARY-:OLD.SALARY;
dbms_output.put_line('Old salary: ' || :OLD.SALARY);
dbms_output.put_line('New salary: ' || :NEW.SALARY);
dbms_output.put_line('Salary difference: ' || sal_diff);
END;