SQL 链接三个表和 SUM

Question

I have three tables, tblPresents, tblPresentsOrdered and tblPresentsDelivered.

What I want to do is sum up all the orders and deliveries for a given present ID, so I can tally up the total ordered and delivered and check for discrepancies.

So far I have the following:

$sql ='SELECT prsName, SUM(ordQuantity) AS qtyOrdered,
SUM(delQuantity) AS qtyDelivered
FROM tblPresentOrders
LEFT JOIN tblPresentDeliveries
ON tblPresentDeliveries.delPresent = tblPresentOrders.ordPresent
RIGHT JOIN tblPresents ON tblPresents.prsID = tblPresentOrders.ordPresent
GROUP BY prsName';

The first column (Ordered) is summing up correctly, but the deliveries is counting the delivery twice (there are two separate orders for that line).

What am I doing wrong?

Answer 1

Because you can have multiple orders per delivery (and presumably multiple presents per order) you need to perform aggregation in derived tables before JOIN ing to avoid duplication in counted/summed values. Note that using a mixture of LEFT JOIN and RIGHT JOIN in the same query can be a bit hard to read so I've rewritten the query using only LEFT JOIN s.

SELECT p.prsName, o.qtyOrdered, d.qtyDelivered
FROM tblPresents p
LEFT JOIN (SELECT ordPresent, SUM(ordQuantity) AS qtyOrdered
           FROM tblPresentOrders
           GROUP BY ordPresent) o ON o.ordPresent = p.prsID
LEFT JOIN (SELECT delPresent, SUM(delQuantity) AS qtyDelivered
           FROM tblPresentDeliveries
           GROUP BY delPresent) d ON d.delPresent = p.prsID