Trigger with table join - sql

Question

I want to create a trigger to update stock inventory. My quantity on stock and quantity ordered by the client are on two different tables. How do i join both?
CREATE OR REPLACE
TRIGGER UPDATE_QUANTITY_TRIGGER
AFTER insert ON SALE_ORDER
FOR EACH ROW
BEGIN
update product values
(:old.product_id,
:old.product_name,
:old.description,
quantity_on_stock-s.quantity,
:old.minimal_quantity,
:old.unit_price,
:old.product_type_id);
from product, sale_order s
where product.product_id=s.product_id;
END;
OR
CREATE OR REPLACE
TRIGGER UPDATE_QUANTITY_TRIGGER
AFTER insert ON SALE_ORDER
FOR EACH ROW
BEGIN
update product set
p.quantity_on_stock= p.quantity_on_stock-s.quantity;
from product p , sale_order s
where p.product_id=s.product_id;
END;
The sale_order table has columns sale_no, quantity, and product_id.

Answer 1

You can't join in an update statement. Your second attempt is closer but is still joining, and has no correlation. You also have a stray semicolon. I think you're looking for something like:
CREATE OR REPLACE
TRIGGER UPDATE_QUANTITY_TRIGGER
AFTER insert ON SALE_ORDER
FOR EACH ROW
BEGIN
update product p set
p.quantity_on_stock = p.quantity_on_stock - :new.quantity
where p.product_id = :new.product_id;
END;
This uses the values from the :new pseudorecord to both identify the product record to be updated, and to get the quantity to decrement the stock level by. There is no need to join to the triggering table again - which isn't allowed anyway.
You may run into unexpected behaviour or results doing this kind of update in a multiuser environment though.