Trigger to enforce constraint that employee never change department - oracle

Question

Suppose I have following schema :
DEPARTMENT (DepartmentName, BudgetCode, OfficeNumber, Phone)
EMPLOYEE (EmployeeNumber, FirstName, LastName, Department, Phone, Email)
Now I need to write a trigger that enforce the constraint that an employee can never change his or her department.
CREATE OR REPLACE TRIGGER department_fixed
BEFORE UPDATE ON EMPLOYEE
FOR EACH ROW
WHEN (old.Department is not null)
BEGIN
dbms_output.put('You can not change department');
END;
/
Is this right way to write this oracle trigger or am missing something ?
Also please help me to write a trigger to allow the deletion of a department if it only has one employee. Assign the last employee to the Human Resources department.

Answer 1

You're close. What this trigger is going to do is for every update of the EMPLOYEE table, it's going to check the original department and if it's not null, it will print something to the screen. I'm assuming you're allowing updates for other fields, right? So my trigger would look like this:
CREATE OR REPLACE TRIGGER EMPLOYEE_BUR
BEFORE UPDATE ON EMPLOYEE
FOR EACH ROW
DECLARE
DEPT_EXCEPTION EXCEPTION; --Declare the exception
PRAGMA EXCEPTION_INIT(DEPT_EXCEPTION, -20002);
L_COUNT NUMBER
BEGIN
SELECT COUNT(*) INTO L_COUNT FROM DEPARTMENT D
WHERE D.DEPARTMENT=:OLD.DEPARTMENT;
IF (:OLD.DEPARTMENT <> :NEW.DEPARTMENT) AND (L_COUNT > 0) THEN
RAISE DEPT_EXCEPTION;
END IF;
END;
.. and then I handle the DEPT_EXCEPTION in my calling procedure. You must either raise an exception -OR- do something with :NEW.DEPARTMENT or else the trigger will finish and the update statement will complete. The "EMPLOYEE_BUR" name is more for best practices, this way you know what it is. "BUR" - "Before Update for each Row"
For the second part, you could have a trigger on DEPARTMENT that fires before the delete to confirm deletion, then one after to reassign the employee:
CREATE OR REPLACE TRIGGER DEPARTMENT_BDR
BEFORE DELETE ON DEPARTMENT
FOR EACH ROW
DECLARE
DEPT_COUNT_EXCEPTION EXCEPTION;
PRAGMA EXCEPTION_INIT(DEPT_COUNT_EXCEPTION, -20003);
L_COUNT NUMBER;
BEGIN
SELECT COUNT(*) INTO L_COUNT FROM EMPLOYEE E
WHERE E.DEPARTMENT=:OLD.DEPARTMENT;
IF (L_COUNT > 1 ) THEN
RAISE DEPT_COUNT_EXCEPTION;
END IF;
END;
Then...
CREATE OR REPLACE TRIGGER DEPARTMENT_ADR
AFTER DELETE ON DEPARTMENT
FOR EACH ROW
BEGIN
UPDATE EMPLOYEE
SET DEPARTMENT='HR'
WHERE EMPLOYEE=:OLD.EMPLOYEE;
END;
So this way, if you're deleting a Department with more than one employee, trigger 2 fires and raises an exception so trigger 3 will not. If trigger 2 completes fine, then trigger 3 will fire which re-assigns the employee. If the delete statement completed, then the update will be allowed since there is no former department.