Question

Let’s say an Order has many Line items and we’re storing the total cost of an order (based on the sum of prices on order lines) in the orders table.

--------------
orders
--------------
id
ref
total_cost
--------------

--------------
lines
--------------
id
order_id
price
--------------

In a simple application, the order and line are created during the same step of the checkout process. So this means

INSERT INTO orders .... 

-- Get ID of inserted order record
INSERT into lines VALUES(null, order_id, ...), ...

where we get the order ID after creating the order record.

The problem I’m having is trying to figure out the best way to store the total cost of an order. I don’t want to have to

1. create an order
2. create lines on an order
3. calculate cost on order based on lines then update record created in 1. in orders table

This would mean a nullable total_cost field on orders for starters…

My solution thus far is to have an order_totals table with a 1:1 relationship to the orders table. But I think it’s redundant. Ideally, since everything required to calculate total costs (lines on an order) is in the database, I would work out the value every time I need it, but this is very expensive.

What are your thoughts?

Answer 1

I will echo the other answers and say that, unless it’s unacceptably expensive to do so, I’d calculate the total cost from the lines table as-and-when required.

Alternatively, one could define triggers that update orders.total_cost as appropriate. However, one would need to define triggers after INSERT, UPDATE and DELETE on lines:

CREATE TRIGGER after_insert_lines AFTER INSERT ON lines FOR EACH ROW
  UPDATE orders SET total_cost = total_cost + NEW.price;

CREATE TRIGGER after_update_lines AFTER UPDATE ON lines FOR EACH ROW
  UPDATE orders SET total_cost = total_cost - OLD.price + NEW.price;

CREATE TRIGGER after_delete_lines AFTER DELETE ON lines FOR EACH ROW
  UPDATE orders SET total_cost = total_cost - OLD.price;