JOIN Multiple Tables with SUM - sql

Question

I need help with the scenario below.
order table
orderId orderAmount
10001 1000.00
10002 2000.00
10003 3000.00
10004 1000.00
10005 1000.00
transfer table
transferId orderId transferAmount
9001 10001 100.00
9002 10001 200.00
9003 10001 25.00
9004 10002 250.00
9005 10002 450.00
9006 10004 100.00
returns table
returnId orderId returnAmount
8001 10001 450.00
8002 10001 50.00
8003 10002 245.00
8004 10003 100.00
Result Needed
/*include all orders from orders table, even if no transfers or returns*/
orderId transfers returns
10001 325.00 500.00 /*sum of transfers and returns per orderId from respective tables*/
10002 700.00 245.00
10003 0.00 100.00
10004 100.00 0.00
10005 0.00 0.00 /*use zero whenever no rows for orderId in respective table*/
How can I implement the above in single query?

Answer 1

This is tricky. I think the easiest way is union all. If I have the arithmetic correct, you want to add the order amount to transfers but not returns:
select orderId,
sum(transfers) as transfers,
sum(returns) as returns
from ((select orderId, Amount, 0 as transfers, 0 as returns
from orders o
) union all
(select orderId, 0, transferAmount, 0
from transfers
) union all
(select orderId, 0, 0, returnAmount
from transfers
)
) otr
group by orderId;

Answer 2

You can just select all the order lines.
For each order, you subquery the tables linked by the OrderId.
SELECT o.orderId
, (SELECT sum(TransferAmount)
FROM Transaction t
WHERE t.OrderId = o.OrderId) 'Transaction'
, (SELECT sum(r.ReturnAmount)
FROM Result r
WHERE r.OrderId = o.OrderId) 'Return'
FROM Order o

Answer 3

I would use derived tables, something like (Apologies if the syntax is a little off, not tested):
SELECT O.OrderID,COALESCE(T.Total,0) AS [Transfer Total],COALESCE(R.Total,0) AS [Return Total]
FROM Orders AS O
LEFT JOIN
(
SELECT T.OrderID,SUM(T.TransferAmount) [Total]
FROM Transfer_Table AS T
GROUP BY T.OrderID
) AS T ON O.OrderID = T.OrderID
LEFT JOIN
(
SELECT R.OrderID,SUM(R.ReturnAmount) [Total]
FROM Return_Table AS R
GROUP BY R.OrderId
) AS R ON R.OrderId = O.OrderID