How to solve mutating table error?

Question

I compiled the trigger below successfully:
CREATE OR REPLACE TRIGGER avail_products
BEFORE INSERT ON orders_tbl
FOR EACH ROW
DECLARE
m_prodqty products_tbl.prod_qty%TYPE;
m_ordered orders_tbl.ord_qty%TYPE;
m_prodid orders_tbl.prod_id%TYPE;
BEGIN
SELECT p.prod_qty, o.ord_qty, o.prod_id
INTO m_prodqty, m_ordered, m_prodid
FROM products_tbl p, orders_tbl o
WHERE ord_num=:NEW.ord_num;
IF m_prodqty>=m_ordered THEN
UPDATE products_tbl
SET prod_qty=prod_qty-m_ordered
WHERE prod_id=m_prodid;
ELSE
RAISE_APPLICATION_ERROR(-20101,'Product is not available');
END IF;
END;
However, when I run my PL/SQL block as follows:
DECLARE
v_max NUMBER;
BEGIN
SELECT MAX(ord_num)+1
INTO v_max
FROM orders_tbl;
INSERT INTO orders_tbl(ord_num, cust_id, prod_id, ord_qty, ord_date, dvy_date, refund)
VALUES (v_max, &cust_id, &prod_id, &ord_qty, '&ord_date', '&dvy_date',0);
COMMIT;
END;
UPDATE orders_tbl
SET refund=0.1*(SELECT price FROM products_tbl WHERE prod_id=(SELECT prod_id FROM orders_tbl WHERE ord_num=(SELECT MAX(ord_num) FROM orders_tbl)))
WHERE ord_num = (SELECT MAX(ord_num) FROM orders_tbl) AND dvy_date>ord_date+3;
COMMIT;
It gives me these errors:
ERROR at line 1:
ORA-01403: no data found
ORA-01403: no data found
ORA-06512: at "SYSTEM.AVAIL_PRODUCTS", line 9
ORA-04088: error during execution of trigger 'SYSTEM.AVAIL_PRODUCTS'
ORA-06512: at line 10
I try to debug it again and again but still can't find the mistake. So I change the trigger option to AFTER INSERT, then I got the error message that the table is mutating and trigger migh not see it. Anyone know how to solve this problem?
ERROR at line 2:
ORA-04091: table SYSTEM.ORDERS_TBL is mutating, trigger/function may not see it
ORA-06512: at "SYSTEM.AVAIL_PRODUCTS", line 13
ORA-04088: error during execution of trigger 'SYSTEM.AVAIL_PRODUCTS'
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Answer 1

This extract from an earlier post might be of help:
You can solve this problem by using following thing
1)create a view on same table with all fields
and write trigger on table (insert,update or delete ) while inserting or updating or deleting row from table read from view.
(Mutating error come when you are reading from one table and want to update,insert or delete row of same table).
2)create a temporary table(but it is possible in 8i onword only) same as table on which you want to write trigger,while updating,inserting or deleting rows of table read from temporary table and after your work is over temporary table auotomatically drop (see proper command in oracle documentation to create temporary table).
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