Sum of last 12 months

I have a table with 3 columns (Year, Month, Value) like this in Sql Server :

I need a SQL request to calculate ValueOfLastTwelveMonths.

SELECT Year,
        Month,
        Value,
        SUM (Value) OVER (PARTITION BY Year, Month)
FROM MyTable

This is much easier if you have a row for each month and year, and then (if needed) you can filter the NULL rows out. The reason it's easier is because then you know how many rows you need to look back at: 11.

If you make a dataset of the years and months, you can then LEFT JOIN to your data, aggregate, and then finally filter the data out:

SELECT *
INTO dbo.YourTable
FROM (VALUES(2021,1,30),
            (2021,2,24),
            (2021,5,26),
            (2021,11,12),
            (2022,1,25),
            (2022,2,40),
            (2022,4,20))V(Year,Month,Value);

GO
WITH YearMonth AS(
    SELECT YT.Year,
           V.Month
    FROM (SELECT DISTINCT Year
          FROM dbo.YourTable) YT
         CROSS APPLY (VALUES(1),(2),(3),(4),(5),(6),(7),(8),(9),(10),(11),(12))V(Month)),
RunningTotal AS(
    SELECT YM.Year,
           YM.Month,
           YT.Value,
           SUM(YT.Value) OVER (ORDER BY YM.Year, YM.Month
                               ROWS BETWEEN 11 PRECEDING AND CURRENT ROW) AS Last12Months
    FROM YearMonth YM
         LEFT JOIN dbo.YourTable YT ON YM.Year = YT.Year
                                   AND YM.Month = YT.Month)
SELECT Year,
       Month,
       Value,
       Last12Months
FROM RunningTotal
WHERE Value IS NOT NULL;

GO
DROP TABLE dbo.YourTable;