Get sum of entries over last 6 months (incomplete months) - sql

Question

My data looks something like this
ProductNumber | YearMonth | Number
1 201803 1
1 201804 3
1 201810 6
2 201807 -3
2 201809 5
Now what I want to have is add an additional entry "6MSum" which is the sum of the last 6 months per ProductNumber (not the last 6 entries).
Please be aware the YearMonth data is not complete, for every ProductNumber there are gaps in between so I cant just use the last 6 entries for the sum. The final result should look something like this.
ProductNumber | YearMonth | Number | 6MSum
1 201803 1 1
1 201804 3 4
1 201810 6 9
2 201807 -3 -3
2 201809 5 2
Additionally I don't want to insert the sum to the table but instead use it in a query like:
SELECT [ProductNumber],[YearMonth],[Number],
6MSum = CONVERT(INT,SUM...)
FROM ...
I found a lot off solutions that use a "sum over period" but only for the last X entries and not for the actual conditional statement of "YearMonth within last 6 months".
Any help would be much appreciated!
Its a SQL Database
EDIT/Answer
It seems to be the case that the gaps within the months have to be filled with data, afterwards something like
sum(Number) OVER (PARTITION BY category
ORDER BY year, week
ROWS 6 PRECEDING) AS 6MSum
Should work.
Reference to the solution : https://dba.stackexchange.com/questions/181773/sum-of-previous-n-number-of-columns-based-on-some-category

Answer 1

You could go the OUTER APPLY route. The following produces your required results exactly:
-- prep data
SELECT
ProductNumber , YearMonth , Number
into #t
FROM ( values
(1, 201803 , 1 ),
(1, 201804 , 3 ),
(1, 201810 , 6 ),
(2, 201807 , -3 ),
(2, 201809 , 5 )
) s (ProductNumber , YearMonth , Number)
-- output
SELECT
ProductNumber
,YearMonth
,Number
,[6MSum]
FROM #t t
outer apply (
SELECT
sum(number) as [6MSum]
FROM #t it
where
it.ProductNumber = t.ProductNumber
and it.yearmonth <= t.yearmonth
and t.yearmonth - it.yearmonth between 0 and 6
) tt
drop table #t

Answer 2

Use outer apply and convert yearmonth to a date, something like this:
with t as (
select t.*,
convert(date, convert(varchar(255), yearmonth) + '01')) as ymd
from yourtable t
)
select t.*, t2.sum_6m
from t outer apply
(select sum(t2.number) as sum_6m
from t t2
where t2.productnumber = t.productnumber and
t2.ymd <= t.ymd and
t2.ymd > dateadd(month, -6, ymd)
) t2;

Answer 3

Just to provide one more option. You can use DATEFROMPARTS to build valid dates from the YearMonth value and then search for values within date ranges.
Testable here: https://rextester.com/APJJ99843
SELECT
ProductNumber , YearMonth , Number
INTO #t
FROM ( values
(1, 201803 , 1 ),
(1, 201804 , 3 ),
(1, 201810 , 6 ),
(2, 201807 , -3 ),
(2, 201809 , 5 )
) s (ProductNumber , YearMonth , Number)
SELECT *
,[6MSum] = (SELECT SUM(number) FROM #t WHERE
ProductNumber = t.ProductNumber
AND DATEFROMPARTS(LEFT(YearMonth,4),RIGHT(YearMonth,2),1) --Build a valid start of month date
BETWEEN
DATEADD(MONTH,-6,DATEFROMPARTS(LEFT(t.YearMonth,4),RIGHT(t.YearMonth,2),1)) --Build a valid start of month date 6 months back
AND DATEFROMPARTS(LEFT(t.YearMonth,4),RIGHT(t.YearMonth,2),1)) --Build a valid end of month date
FROM #t t
DROP TABLE #t

Answer 4

So a working query (provided by a colleauge of mine) can look like this
SELECT [YearMonth]
,[Number]
,[ProductNumber]
, (Select Sum(Number) from [...] DPDS_1 where DPDS.ProductNumber =
DPDS_1.ProductNumber and DPDS_1.YearMonth <= DPDS.YearMonth and DPDS_1.YearMonth >=
convert (int, left (convert (varchar, dateadd(mm, -6, DPDS.YearMonth + '01'), 112),
6)))FROM [...] DPDS