Deleting data from a SQL table after it is altered in another

You probably need an UPDATE rather than an DELETE. That being said, I would not advise to update the stock after having accepted the order. I feel it more manageable the other way around: update the stock, and accept the order only if there was enough items.

Something like that:

--> client place an order for 20 item id XXXX:

UPDATE ProductType SET amount = amount - 20
 WHERE amount >= 20 AND item_id = 'XXXX';
 ^^^^^^^^^^^^^^^^^^
Please note that "guard" clause

That statement will either update 0 or 1 row. 1 row means "ok, stock updated". 0 means "not enough items left in stock". This is especially important in multi-user environment where you can have several concurrent updates of your stock for the same item.

After that only:

--> If 1 row updated

INSERT INTO OrderItem(..., amount) VALUES (....., 20);

Finally, in a real world application, you need to wrap all those statements in a transaction in case of unexpected failure after the stock update.

I dont know correctly what you are expecting.Assuming

If table ProductType --> type1 amount

Then inserting to table OrderItem n items of type1.So assuming that you have to decrease that much amount from ProductType

When you insert n amount into OrderItem,

Here you can use UPDATE

UPDATE OrderItem SET column_name = column_name - n WHERE ...

Update

update the stock, and accept the order only if there was enough items, is the good process steps.

Hope this helps