Converting Multiple subqueries with GROUP BY to JOIN - mysql

Question

I'm working on a simple ordering system in MySQL and I came across this snag that I'm hoping some SQL genius can help me out with.
I have a table for Orders, Payments (with a foreign key reference to the Order table), and OrderItems (also, with a foreign key reference to the Order table) and what I would like to do is get the total outstanding balance (Total and Paid) for the Order with a single query. My initial thought was to do something simple like this:
SELECT Order.*, SUM(OrderItem.Amount) AS Total, SUM(Payment.Amount) AS Paid
FROM Order
JOIN OrderItem ON OrderItem.OrderId = Order.OrderId
JOIN Payment ON Payment.OrderId = Order.OrderId
GROUP BY Order.OrderId
However, if there are multiple Payments or multiple OrderItems, it messes up Total or Paid, respectively (eg. One OrderItem record with an amount of 100 along with two Payment Records will produce a Total of 200).
In order to overcome this, I can use some subqueries in the following way:
SELECT Order.OrderId, OrderItemGrouped.Total, PaymentGrouped.Paid
FROM Order
JOIN (
SELECT OrderItem.OrderId, SUM(OrderItem.Amount) AS Total
FROM OrderItem
GROUP BY OrderItem.OrderId
) OrderItemGrouped ON OrderItemGrouped.OrderId = Order.OrderId
JOIN (
SELECT Payment.OrderId, SUM(Payment.Amount) AS Paid
FROM Payment
GROUP BY Payment.OrderId
) PaymentGrouped ON PaymentGrouped.OrderId = Order.OrderId
As you can imagine (and as an EXPLAIN on this query will show), this is not exactly an optimal query so, I'm wondering, is there any way to convert these two subqueries with GROUP BY statements into JOINs?

Answer 1

The following is likely to be faster with the right indexes:
select o.OrderId,
(select sum(oi.Amount)
from OrderItem oi
where oi.OrderId = o.OrderId
) as Total,
(select sum(p.Amount)
from Payment p
where oi.OrderId = o.OrderId
) as Paid
from Order o;
The right indexes are OrderItem(OrderId, Amount) and Payment(OrderId, Amount).
I don't like writing aggregation queries this way, but it can sometimes help performance in MySQL.

Answer 2

Some answers have already suggested using a correlated subquery, but have not really offered an explanation as to why. MySQL does not materialise correlated subqueries, but it will materialise a derived table. That is to say with a simplified version of your query as it is now:
SELECT Order.OrderId, OrderItemGrouped.Total
FROM Order
JOIN (
SELECT OrderItem.OrderId, SUM(OrderItem.Amount) AS Total
FROM OrderItem
GROUP BY OrderItem.OrderId
) OrderItemGrouped ON OrderItemGrouped.OrderId = Order.OrderId;
At the start of execution MySQL will put the results of your subquery into a temporary table, and hash this table on OrderId for faster lookups, whereas if you run:
SELECT Order.OrderId,
( SELECT SUM(OrderItem.Amount)
FROM OrderItem
WHERE OrderItem.OrderId = OrderId
) AS Total
FROM Order;
The subquery will be executed once for each row in Order. If you add something like WHERE Order.OrderId = 1, it is obviously not efficient to aggregate the entire OrderItem table, hash the result to only lookup one value, but if you are returning all orders then the inital cost of creating the hash table will make up for itself it not having to execute the subquery for every row in the Order table.
If you are selecting a lot of rows and feel the materialisation will be of benefit, you can simplifiy your JOIN query as follows:
SELECT Order.OrderId, SUM(OrderItem.Amount) AS Total, PaymentGrouped.Paid
FROM Order
INNER JOIN OrderItem
ON OrderItem.OrderID = Order.OrderID
INNER JOIN
( SELECT Payment.OrderId, SUM(Payment.Amount) AS Paid
FROM Payment
GROUP BY Payment.OrderId
) PaymentGrouped
ON PaymentGrouped.OrderId = Order.OrderId;
GROUP BY Order.OrderId, PaymentGrouped.Paid;
Then you only have one derived table.

Answer 3

What about something like this:
SELECT Order.OrderId, (
SELECT SUM(OrderItem.Amount)
FROM OrderItem as OrderItemGrouped
where
OrderItemGrouped.OrderId = Order.OrderId
), AS Total,
(
SELECT SUM(Payment.Amount)
FROM Payment as PaymentGrouped
where
PaymentGrouped.OrderId = Order.OrderId
) as Paid
FROM Order
PS: You win again #Gordon xD

Answer 4

Select o.orderid, i.total, s.paid
From orders o
Left join (select orderid, sum(amount)
From orderitem) i
On i.orderid = o.orderid
Ieft join (select orderid, sum(amount)
From payments) s
On s.orderid = o.orderid