TSQL - Query to get sum of last 3 months QTY - sql-server

Question

I am working on a query to get sum of (current + last 2 month) QTY based on a date field in the table.
CREATE table #temp_Sales
( Client varchar(10),
Sale_Month Date,
Qty int)
Insert into #temp_Sales VALUES
( 'AAAA', '2022-06-01', 5 ),
( 'AAAA', '2022-05-01', 10 ),
( 'AAAA', '2022-05-01', 2 ),
( 'AAAA', '2022-04-01', 5 ),
( 'AAAA', '2022-02-01', 15),
( 'BBBB', '2022-05-01', 2 ),
( 'BBBB', '2022-04-01', 4),
( 'BBBB', '2022-03-01', 6 ),
( 'BBBB', '2022-03-01', 10 ),
( 'BBBB', '2022-01-01', 6 ),
( 'BBBB', '2021-10-01', 10),
( 'BBBB', '2021-09-01', 2 ),
( 'BBBB', '2021-11-01', 4 ),
( 'BBBB', '2021-08-01', 6),
( 'BBBB', '2021-07-01', 8 ),
( 'CCCC', '2021-11-01', 2 ),
( 'CCCC', '2021-10-01', 3 ),
( 'CCCC', '2021-09-01', 30 ),
( 'CCCC', '2021-06-01', 4 )
Sample data:
Expected Output:
The Sale_month is not consecutive and same month can appear more than once for a client in the table.
Example : For the Client AAAA and Sale Month 2022-06-01 the qty should include the sum(QTY) of current and last 2 months ( 2022-06-01,2022-05-01 and 2022-04-01) for that client. QTY = 5 + 10 + 2 + 5 = 22
For the client BBBB and Sale month 2022-03-01 . QTY = 6 + 10 + 6 = 22
;With da AS
(SELECT *, DATEADD(MM,-2,Sale_month)as last_two_Months FROM #temp_Sales)
Select Client,Sale_month,Sum(qty) from da
WHERE Sale_month Between last_two_Months and Sale_month
GROUP BY Client,Sale_month
Order by client
Tried the above query. But not working as expected not sure how to group by using last_two_Months and Sale_month. Any help is much appreciated.

Answer 1

Here is one way you could tackle this.
select Sale_Month
, client
, Last3Months = max(x.PeriodSales)
from #temp_Sales s
cross apply
(
select PeriodSales = sum(Qty)
from #temp_Sales s2
where s2.Sale_Month >= dateadd(month, -2, s.Sale_Month)
and s2.Sale_Month <= s.Sale_Month
and s2.Client = s.Client
) x
group by Sale_Month
, client
order by Client
, Sale_Month desc

Answer 2

group on qty to eliminate duplicates, then join to self like this:
;WITH CTE AS(
select
[Client],
[Sale_Month],
SUM(QTY) QTY
from #tmp
group by
[Client],
[Sale_Month]
)
select a.[Client], a.[Sale_Month], SUM(b.QTY) QTY
from
CTE a
INNER JOIN CTE b on
a.Client = b.Client and
DATEDIFF(mm, b.[Sale_Month], a.[Sale_Month]) between 0 and 2
group by
a.[Client], a.[Sale_Month]

Answer 3

Firstly, if you don't already have a Calendar Table invest in one.
After that, you can create the data set you need by JOINing data from your Sales table to your calendar table, then getting the sales via a LEFT JOIN and aggregating them. Then you can use a windowed SUM for the last 3 months. Finally, as you seem to not want months that don't exist, filter those back out:
WITH Clients AS(
SELECT Client,
MIN(Sale_Month) AS MinMonth,
MAX(Sale_Month) AS MaxMonth
FROM dbo.Sales
GROUP BY Client),
ClientDates AS(
SELECT C.Client,
CT.CalendarDate
FROM Clients C
JOIN dbo.CalendarTable CT ON C.MinMonth <= CT.CalendarDate
AND C.MaxMonth >= CT.CalendarDate
WHERE CT.CalenderDay = 1),
ClientSales AS(
SELECT CD.Client,
CD.CalendarDate,
SUM(S.Qty) AS Qty
FROM ClientDates CD
LEFT JOIN dbo.Sales S ON CD.Client = S.Client
AND CD.CalendarDate = S.Sale_Month
GROUP BY CD.Client,
CD.CalendarDate),
Last3 AS(
SELECT CS.Client,
CS.CalendarDate,
CS.Qty,
SUM(CS.Qty) OVER (PARTITION BY CS.Client ORDER BY CS.CalendarDate
ROWS BETWEEN 2 PRECEDING AND CURRENT ROW) AS Last3Months
FROM ClientSales CS)
SELECT L3.Client,
L3.CalendarDate AS Sale_Month,
L3.Last3Months
FROM Last3 L3
WHERE L3.Qty IS NOT NULL
ORDER BY Client,
Sale_Month DESC;