-- 6.2
SELECT	DISTINCT CustomerID Klient,
		YEAR(OrderDate) Rok,
		COUNT(SalesOrderID) OVER (PARTITION BY (CustomerID) ORDER BY Year(OrderDate)) AS 'Liczba zamówień'
FROM Sales.SalesOrderHeader
ORDER BY 1, 3, 2;

-- 6.3
SELECT	SalesPersonID,
		Rok,
		Miesiac,
		"W miesiącu",
		"W roku",
		"W roku narastająco",
		SUM("W miesiącu") OVER (PARTITION BY SalesPersonID, Rok ORDER BY Miesiac ROWS BETWEEN 1 PRECEDING AND CURRENT ROW) AS "Obecny i poprzedni miesiąc"
FROM ( SELECT DISTINCT SalesPersonID,
				YEAR(OrderDate) AS Rok,
				MONTH(OrderDate) AS Miesiac,
				COUNT(SalesOrderID) OVER (PARTITION BY SalesPersonID, YEAR(OrderDate), MONTH(OrderDate)) AS "W miesiącu",
				COUNT(SalesOrderID) OVER (PARTITION BY SalesPersonID, YEAR(OrderDate)) AS "W roku",
				COUNT(SalesOrderID) OVER (PARTITION BY SalesPersonID, YEAR(OrderDate) ORDER BY MONTH(OrderDate)) AS "W roku narastająco"
		FROM Sales.SalesOrderHeader
		WHERE SalesPersonID IS NOT NULL
) sourceTable
ORDER BY 1, 2, 3;