Oracle trigger conflict with null - sql

Question

First and foremost, this is for an assignment, so most of the really cool pre-written functions you'll want to suggest, I will not be allowed to use.
I have a couple tables that all have fields
creation_date
created_by
last_update_date
last_updated_by
I need to write a trigger that fills these in for creation or updating. The problem is, these tables have null values that are problematic to me. Example:
CREATE TABLE parts
(
pno NUMBER,
pname VARCHAR2(50) NOT NULL,
qoh NUMBER NOT NULL,
price NUMBER(5,2),
reorder_level NUMBER(2),
creation_date DATE NOT NULL,
created_by VARCHAR2(10) NOT NULL,
last_update_date DATE NOT NULL,
last_updated_by VARCHAR2(10) NOT NULL,
CONSTRAINT parts_PK PRIMARY KEY (pno))
These NOT NULL's are given to me and I'm not allowed to change them. So I'm having trouble conceptualizing this.
If my trigger adds these values before the field is created, I can't do an INSERT INTO with those fields blank because they're NOT NULL.
If my trigger adds these values after the field is created, I get compilation errors ORA-00903 and 00922. Invalid table name and invalid option.
I was thinking my trigger would look like
CREATE OR REPLACE TRIGGER pcreate
BEFORE UPDATE on parts
FOR EACH ROW
BEGIN
UPDATE
SET creation_date = SYSDATE;
SET created_by = USER;
SET last_update_date = SYSDATE;
SET last_updated_by = USER;
END;
/
CREATE OR REPLACE TRIGGER pchange
BEFORE UPDATE on parts
FOR EACH ROW
BEGIN
UPDATE
SET last_update_date = SYSDATE;
SET last_updated_by = USER;
END;
/
repeat for the other tables
I might be allowed to use UPSERT but I don't really know how that works.. Any suggestions are welcome. I'm really in this to learn so any other advice is appreciated.
EDIT:
My package that will not acknowledge the trigger is as follows. Do I need to call the trigger inside the package?
CREATE OR REPLACE PACKAGE process_orders
AS
PROCEDURE add_order (p_cno IN NUMBER, p_eno IN NUMBER, p_received IN DATE);
PROCEDURE add_order_details (p_ono IN NUMBER, p_pno IN NUMBER, p_qty IN NUMBER);
PROCEDURE ship_order (p_ono IN NUMBER, p_sdate IN DATE);
PROCEDURE delete_order (p_ono IN NUMBER);
FUNCTION total_emp_sales (f_eno IN NUMBER) RETURN NUMBER;
END process_orders;
/
CREATE OR REPLACE PACKAGE BODY process_orders
AS
PROCEDURE add_order (p_cno IN NUMBER, p_eno IN NUMBER, p_received IN DATE)
IS
ao_emsg VARCHAR2(100);
p_rec_today DATE;
BEGIN
SELECT SYSDATE INTO p_rec_today FROM dual;
IF p_received is null THEN
INSERT INTO orders (ono, cno, eno, received)
VALUES(order_number_seq.NEXTVAL,p_cno,p_eno,p_rec_today);
ELSE
INSERT INTO orders (ono, cno, eno, received)
VALUES(order_number_seq.NEXTVAL,p_cno,p_eno,p_received);
END IF;
EXCEPTION
WHEN OTHERS THEN
ao_emsg := substr(SQLERRM,1,100);
INSERT INTO orders_errors (ono,transaction_date,message)
VALUES(order_number_seq.CURRVAL,SYSDATE,ao_emsg);
END;
--Etc. Procedures
END;
/

Answer 1

In your trigger, you simply want to modify the :new record. Your triggers would look something like
CREATE OR REPLACE TRIGGER parts_before_insert
BEFORE INSERT on parts
FOR EACH ROW
BEGIN
:new.creation_date := SYSDATE;
:new.created_by := USER;
:new.last_update_date := SYSDATE;
:new.last_updated_by := USER;
END;
and
CREATE OR REPLACE TRIGGER parts_before_update
BEFORE UPDATE on parts
FOR EACH ROW
BEGIN
:new.last_update_date := SYSDATE;
:new.last_updated_by := USER;
END;
In your INSERT statement, you would omit these four columns and let the trigger fill in the values. For example (obviously, your primary keys would be coming from something like a sequence rather than being hard-coded)
SQL> insert into parts( pno, pname, qoh, price, reorder_level )
2 values( 1, 'Widget', 10, 100, 75 );
1 row created.
SQL> select *
2 from parts;
PNO PNAME QOH
---------- -------------------------------------------------- ----------
PRICE REORDER_LEVEL CREATION_ CREATED_BY LAST_UPDA LAST_UPDAT
---------- ------------- --------- ---------- --------- ----------
1 Widget 10
100 75 26-SEP-12 SCOTT 26-SEP-12 SCOTT

Answer 2

There is no need of executing UPDATE statement in triggers in your case. Just assign new values to the columns using :new.
CREATE OR REPLACE TRIGGER pchange
BEFORE UPDATE on parts
FOR EACH ROW
BEGIN
:new.last_update_date = SYSDATE;
:new.last_updated_by = USER;
END;