Oracle: After update Trigger

Question

I have 3 tables:

Category(CategoryId, Name)
Product(ProductId, Name, Description, CategoryId)
OrderItem(OrderId, OrdinalNumber, ProductId, CategoryId)

I want to create an AFTER UPDATE trigger that changes CategoryId (based on new ProductId) in OrderItem after update of ProductId in OrderItem .

Can somebody help with this trigger?

Answer 1

Duplicating the category ID in the order line isn't something you'd usually want to do, but if you're set on that, you need a 'before' trigger, not an 'after' one - since you need to change a value in the row being updated:

create or replace trigger orderitem_cat_trig
before insert or update on orderitem
for each row
begin
  select categoryid
  into :new.categoryid
  from product
  where productid = :new.productid;
end;
/

I've made it both insert and update on the assumption you'll want to set the value for new order items too.

Answer 2

Unless you like database deadlocks, general performance issues, data corruption and unpredictable results, this type of updates is not advisable. If your performance is a problem, check indexes and queries. Do not replicate your columns in tables, especially not when they're part of an foreign key. I'm not the dogmatic type, but in this case I will not budge ;-)