SQL Server : query with incorrect SUM result - sql

Question

My SQL Server query is supposed to get a count of each customer's number of orders, and the SUM of their reward points. For most customers the result is accurate (most people only have one or two orders). For a few people, the result is wildly off.
Here's the original query:
SELECT
c.email,
c.lastlogindate,
c.custenabled,
c.maillist,
d.GroupName,
COUNT(o.orderid) AS orders,
SUM(r.points) AS total_points
FROM
((customers c
LEFT JOIN orders o ON (c.contactid = o.ocustomerid AND o.ostep = 'step 5')
)
LEFT JOIN discount_group d ON c.discount = d.id
)
LEFT JOIN
customer_rewards r ON r.contactid = c.contactid
WHERE
c.last_update > '2014-02-01'
OR c.lastlogindate > '2014-02-01'
GROUP BY
c.email, c.custenabled, c.maillist, c.lastlogindate, d.GroupName;
For one example, customerid 1234 has placed 21 orders, totaling 2724 points. This will report that he has placed 441 orders (21 * 21) valued at 57204 points (2724 * 21). The raw data is fine, but each order row is being duplicated by the amount of orders they placed (but not for most customers...)
If I change the query to this:
SELECT
o.orderid,
c.email,
COUNT(o.orderid) AS orders,
SUM(r.points) AS total_points
FROM
((customers c
INNER JOIN orders o ON (c.contactid = o.ocustomerid AND o.ostep = 'step 5')
)
)
INNER JOIN
customer_rewards r ON r.contactid = c.contactid
WHERE
c.last_update > '2014-02-01'
OR c.lastlogindate > '2014-02-01'
GROUP BY
c.email, o.orderid;
The aggregate functions are calculated properly, but it will display one result for each order placed. So it will show "Customer 1234/21 orders/2724 points", 21 times.
I did remove the 'discount_group' join in the second query, but that was just to make it easier to read and change. That hasn't had any effect on results.

Answer 1

Here is a solution using common table expressions to aggregate your results.
Note: this will not show customers that have 0 orders or 0 rewards points. If you would like to show these, change the INNER JOINs to LEFT JOINs
WITH cteOrders AS
(
SELECT o.ocustomerid, orderCount = count(*)
FROM orders o
WHERE o.ostep = 'step 5'
GROUP BY o.ocustomerid
)
, cteRewards as
(
SELECT cr.contactid, total_points = SUM(cr.points)
FROM customer_rewards cr
GROUP BY cr.contactid
)
SELECT
c.email,
o.orderCount as orders,
r.total_points
FROM
customers c
INNER JOIN cteOrders o ON c.contactid = o.ocustomerid
INNER JOIN cteRewards r ON r.contactid = c.contactid
WHERE
c.last_update > '2014-02-01'
OR c.lastlogindate > '2014-02-01'
;
Or using subqueries:
SELECT
c.email,
o.orderCount as orders,
r.total_points
FROM
customers c
INNER JOIN
(
SELECT o.ocustomerid, orderCount = count(*)
FROM orders o
WHERE o.ostep = 'step 5'
GROUP BY o.ocustomerid
) o ON c.contactid = o.ocustomerid
INNER JOIN
(
SELECT cr.contactid, total_points = SUM(cr.points)
FROM customer_rewards cr
GROUP BY cr.contactid
) r ON r.contactid = c.contactid
WHERE
c.last_update > '2014-02-01'
OR c.lastlogindate > '2014-02-01'
;