SELECT Year, Month, OrdersNum M_Orders, 
            OrdersNum - FIRST_VALUE(OrdersNum) 
                                 OVER(ORDER BY Year, Month)  AS Diff_M_Orders
FROM (
            SELECT calendaryear AS Year, monthnumberofyear AS Month, 
                        COUNT(DISTINCT orderkey) AS OrdersNum
            FROM factonlinesales fos INNER JOIN dimdate dd
                ON fos.orderdatekey = dd.datekey
            INNER JOIN dimsalesterritory dst 
                ON dst.salesterritorykey = fos.salesterritorykey
            WHERE calendaryear = 2024 AND dst.salesterritoryname = 'EMEA'
            GROUP BY calendaryear, monthnumberofyear
            )
ORDER BY 1, 2 ;