Logic constraint between different tables in SQL

Question

Consider I have three tables: order , sub_order and product . The cost of an sub_order is built upon a complex formula which then depends on the individual costs of the products. The cost of the order is simply the sum of the sub_order costs, although this formula might change in the future.

We store the derived field order.order_cost for convinience.

The questions I have are:

Should business rules be applied to the database layer? If so, is there a way to force the constraint for order_cost using SQL? That is, order_cost is always the sum of sub_order_cost

Answer 1

If you want the order cost to always be the sum of the costs from sub_order , then calculate the value on the fly. There is no need to store the value, unless you have a performance reason. Your question doesn't mention performance as a reason to duplicate the data.

If you want the order cost to be initialized to the sum, then you can use a trigger. That would be strange, though, because I would expect the order to be created before the sub_order s.

If you want to maintain the order amount as sub_order s are created, deleted, and updated, then you need triggers. Or, just calculate the values on the fly.