带有INSERT的Oracle SQL事务

Question

I am writing a database of warehouse of groceries. I have tables clients, workers, orders, products and some which are not really important in that question.

I want to avoid a problem where client makes a new order for product but there isn't enough amount of this product in our warehouse.

I have to make a transaction which avoid situation when two clients are making a new order at the same time and one of them ordered for example the last item while making order by the second client.

I don't really know how to do it because only workers can decrease amount of every product. So I want so that while deleting a order (which was made by client) by worker the amount of available product was decreased.

I know that I can put it into trigger but the question is:

how to define the transaction of ordering product with checking available amount. Should I count the difference between avalaible amount in table Products and amount of ordered product by the time(unrealized orders).

How to do it ?

Answer 1

对于订单，您必须管理该产品的库存，每当有人要下订单时，您都必须检查产品的可用性。

Answer 2

One thing you could do is place a trigger on the orders table, that subtracts quantity from the products table and raises an exception if the amount is negative. The trigger would have like:

update products set current_quantity = current_quantity - :new.amount_purchased
 where id = :new.product_id
returning current_quantity into l_current_quantity;

if l_current_quantity < 0 then
    raise_application_error( -20001, 'Amount ordered larger than remaining quantity.' );
end if;

You'll have to handle the exception in your ordering procedure.