GROUP BY & SUM of values with missing MONTHS - sql

Question

I have gone through a lot of examples and joined couple of them in order to come down to the following statement;
DECLARE #StartDate SMALLDATETIME, #EndDate SMALLDATETIME;
SELECT #StartDate = '20170930', #EndDate = '20180930';
;WITH d(d) AS
(
SELECT DATEADD(MONTH, n, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0))
FROM ( SELECT TOP (DATEDIFF(MONTH, #StartDate, #EndDate) + 1)
n = ROW_NUMBER() OVER (ORDER BY [object_id]) - 1
FROM sys.all_objects ORDER BY [object_id] ) AS n
)
SELECT
[Period] = CONVERT(VARCHAR(4), YEAR(d.d)) + '-' + CONVERT(VARCHAR(2), MONTH(d.d)),
QtyTotal = ISNULL(SUM(o.QEXIT),0)
FROM d LEFT OUTER JOIN VE_STOCKTRANS AS o
ON o.TRANSDATE >= d.d
AND o.TRANSDATE < DATEADD(MONTH, 1, d.d)
WHERE STOCKID = 6000 AND TRANSTYPE = 3553
GROUP BY d.d
ORDER BY d.d;
I need to get the total sales quaantity of an item for the past year. If the item does not have any sales for that particular month, 0 should be displayed next to that month. The above query does what is required unless the WHERE clause is provided. As soon as I add the WHERE clause to get the data for a specific product, the months with no sales dissappears.
I would be grateful if an experienced SQL developer can show me the right direction on this.
Thanks

Answer 1

You need to move condtition to ON:
-- ...
SELECT
[Period] = CONVERT(VARCHAR(4),YEAR(d.d)) +'-'+ CONVERT(VARCHAR(2), MONTH(d.d)),
QtyTotal = ISNULL(SUM(o.QEXIT),0)
FROM d LEFT OUTER JOIN VE_STOCKTRANS AS o
ON o.TRANSDATE >= d.d
AND o.TRANSDATE < DATEADD(MONTH, 1, d.d)
AND STOCKID = 6000 AND TRANSTYPE = 3553 -- here
GROUP BY d.d
ORDER BY d.d;

Answer 2

A more generic approach is to apply the filter before you join.
;WITH d(d) AS
(
SELECT DATEADD(MONTH, n, DATEADD(MONTH, DATEDIFF(MONTH, 0, #StartDate), 0))
FROM ( SELECT TOP (DATEDIFF(MONTH, #StartDate, #EndDate) + 1)
n = ROW_NUMBER() OVER (ORDER BY [object_id]) - 1
FROM sys.all_objects ORDER BY [object_id] ) AS n
),
o AS
(
SELECT *
FROM VE_STOCKTRANS
WHERE STOCKID = 6000
AND TRANSTYPE = 3553
)
SELECT
[Period] = CONVERT(VARCHAR(4), YEAR(d.d)) + '-' + CONVERT(VARCHAR(2), MONTH(d.d)),
QtyTotal = ISNULL(SUM(o.QEXIT),0)
FROM
d
LEFT OUTER JOIN
o
ON o.TRANSDATE >= d.d
AND o.TRANSDATE < DATEADD(MONTH, 1, d.d)
GROUP BY
d.d
ORDER BY
d.d;
It's not strictly necessary here, as you've seen in the other answer. When doing FULL OUTER JOIN or other complex queries, however, it can be extremely helpful to filter your sources in one scope and join in a separate scope.
(I always filter my sources, I hate lumpy ketchup.)