How do I create the update statement that checks if a vlue is lower than the reorder_point attribute for a product - oracle

Question

Utilize the Outdoor Clubs & Product database to create a trigger “product_reorder_au” that is associated with an update operation on the product table. The trigger checks to see whether during the update of the quantity_in_stock attribute, if its value gets lower than the reorder_point attribute value for a product. When this situation occurs, the trigger automatically inserts a new purchase order in the purchase_order table. The new purchase order will use the existing supplier_no attribute value for the product in the product table, and the quantity attribute value will be same as the reorder_qty value for the product in the product table. Save the trigger source as a script file.
This is what the product table looks like
So far this is what I have but I am just generally confused on how to update the table and add in what I am guessing would be an if then statement to only update when the reorderpoint is lower than the quantitiy_in_stock.
CREATE TRIGGER product_reorder_au
AFTER UPDATE OF product
for each row
begin
IF Quantity_In_Stock < Reorder_Point
THEN
INSERT INTO Purchase_Order (Po_No, Po_Date, Product_Id, Quantity, Supplier_id)
VALUES ( );
END IF;
END;
--Test Script
update product
set quantity_in_stock = 5
where product_id = 10012;
Attached in the other tables

Answer 1

You need a BEFORE trigger not AFTER trigger. See below:
--Table Preparation
CREATE TABLE product_1 (
quantity_in_stock NUMBER,
reorder_point NUMBER,
product_id NUMBER,
supplier_id NUMBER,
reorder_qty NUMBER
);
INSERT INTO product_1 VALUES (
20,
5,
10010,
500,
25
)
CREATE TABLE purchase_order (
po_no NUMBER,
po_date DATE,
product_id NUMBER,
quantity NUMBER,
supplier_id NUMBER
);
SQL> Select * from product_1;
QYT_IN_STOCK REORDER_PNT PRDT_ID SUPP_ID RERDR_QTY
----------- ---- ----- ----- -----
4 5 10010 500 25
--Trigger
CREATE TRIGGER product_reorder_au BEFORE
UPDATE ON product_1
FOR EACH ROW
WHEN ( new.quantity_in_stock < old.reorder_point )
BEGIN
INSERT INTO purchase_order (
po_no,
po_date,
product_id,
quantity,
supplier_id
) VALUES (
1, --Populate this coulmn by a sequence or by some logic
SYSDATE,
:old.product_id,
:old.reorder_qty,
:old.supplier_id
);
END;
Execution:
SQL> UPDATE product_1
SET
quantity_in_stock = 4
WHERE product_id = 10010;
SQL> SELECT * FROM purchase_order;
PO_NO PO_DATE PRODUCT_ID QTY SUPP_ID
----- ------- -------- ---- -------
1 25-10-18 10010 25 500