SELECT region, rok, miesiąc, liczba_klientow, narastajaco, RANK() OVER (ORDER BY liczba_klientow DESC)
FROM
(

SELECT DISTINCT T.CountryRegionCode region , YEAR(OrderDate) rok, MONTH(OrderDate) miesiąc,
COUNT(C.CustomerID) OVER (PARTITION BY T.CountryRegionCode, YEAR(OrderDate), MONTH(OrderDate)) liczba_klientow,
COUNT(C.CustomerID) OVER (PARTITION BY T.CountryRegionCode, YEAR(OrderDate) ORDER BY MONTH(OrderDate)) narastajaco
FROM Sales.SalesOrderDetail AS SOD
JOIN Sales.SalesOrderHeader AS SOH
ON SOD.SalesOrderID = SOH.SalesOrderID
JOIN Sales.Customer AS C
ON SOH.CustomerID = C.CustomerID
JOIN Sales.SalesTerritory AS T
ON C.TerritoryID = T.TerritoryID
) AS Podzap

ORDER BY 1,2,3