Left join returning bad values - sql

Question

I'm not very good with SQL queries but I attempted to write this one:
SELECT DATEPART(YY,Orders.OrderDate) as Year,
DATEPART(MM,Orders.OrderDate) as Month,
(SUM(case when OrderDetails.ProductCode = 'XXX' then
OrderDetails.ProductPrice else 0 end) + SUM(Orders.Total))
AS XXX
FROM Orders
LEFT JOIN OrderDetails ON Orders.OrderID = OrderDetails.OrderID
WHERE Orders.OrderStatus = 'Shipped'
GROUP BY DATEPART(MM,Orders.OrderDate), DATEPART(YY,Orders.OrderDate)
ORDER BY DATEPART(YY,Orders.OrderDate),DATEPART(MM,Orders.OrderDate)
The OrderDetails is linked to the Orders table by the field OrderID. In this SELECT query I'm trying to get the SUM of OrderDetails.ProductPrice when the OrderDetails.ProductCode is XXX and add it to the Orders.Total to get total amounts for each month/year.
The query is working except for one problem (that's probably either a amateur mistake or has been worked around several times). When performing the LEFT JOIN, the OrderDetails table can have multiple records linked to the Orders table which is throwing bad results in the SUM(Orders.Total). I've isolated that issue I just can't seem to fix it.
Can anybody point me in the right direction?

Answer 1

If we assume that the XXX product only appears at most once for each order, then this should work:
SELECT year(o.OrderDate) as Year, month(o.OrderDate) as Month,
(COALESCE(SUM(od.ProductPrice), 0) + SUM(o.Total)) AS XXX
FROM Orders o LEFT JOIN
OrderDetails od
ON o.OrderID = od.OrderID AND od.ProductCode = 'XXX'
WHERE o.OrderStatus = 'Shipped'
GROUP BY year(o.OrderDate), month(o.OrderDate)
ORDER BY year(o.OrderDate), month(o.OrderDate);
If it can appear multiple times, then move that part of the aggregation to a subquery:
SELECT year(o.OrderDate) as Year, month(o.OrderDate) as Month,
(COALESCE(XXX, 0) + SUM(o.Total)) AS XXX
FROM Orders o LEFT JOIN
(SELECT od.OrderId, SUM(od.ProductPrice) as XXX
FROM OrderDetails od
WHERE od.ProductCode = 'XXX'
GROUP BY od.OrderId
) od
ON o.OrderID = od.OrderID
WHERE o.OrderStatus = 'Shipped'
GROUP BY year(o.OrderDate), month(o.OrderDate)
ORDER BY year(o.OrderDate), month(o.OrderDate);