SELECT
      si.MdtId
    , m.MdtNumber
    , m.MdtName
    , s.StoreNumber
    , s.StoreName
    , YEAR(si.StartDate) AS Jahr
    , MONTH(si.StartDate) AS Monat
    , (SELECT COUNT(si2.IdStockItem) FROM tblStockItem si2 WHERE si2.MdtId = si.MdtId AND si2.StoreId = si.StoreId
        AND YEAR(si2.StartDate) = YEAR(si.StartDate) AND MONTH(si.StartDate) = MONTH(si2.StartDate)) AS [ZurückUmschläge]
    , (SELECT CASE WHEN SUM(Amount) IS NULL THEN 0 ELSE SUM(Amount) END FROM tblEnvelope e WHERE e.MdtID = si.MdtId AND e.StoreID = si.StoreId
        AND YEAR(e.DeliveryDate) = YEAR(si.StartDate) AND MONTH(e.DeliveryDate) = MONTH(si.StartDate)) AS [geliefertumschläge]
FROM tblStockItem si
    JOIN tblMandant m ON si.MdtId = m.IdMdt
    JOIN tblStores s ON s.IdStore = si.StoreId
WHERE YEAR(si.StartDate)>= 2014 AND si.MdtId = 79 AND si.StoreId = 2610
GROUP BY si.MdtId, m.MdtNumber, m.MdtName, s.StoreNumber, s.StoreName, YEAR(si.StartDate), MONTH(si.StartDate), si.StoreId
ORDER BY YEAR(si.StartDate), MONTH(si.StartDate)