Thank you Jeff, great!
Cheers,
Julian

(https://www.red-gate.com/simple-talk/sql/t-sql-programming/sql-server-apply-basics/)


DECLARE @IDHUIS VARCHAR(4)
DECLARE @IDAFD VARCHAR(4)
DECLARE @JAAR INT
DECLARE @WEEKBEGIN INT
DECLARE @WEEKEND INT

SET @IDHUIS = 'XX'
SET @IDAFD = 'A03'
SET @JAAR = 2017
SET @WEEKBEGIN = 1
SET @WEEKEND = 52

SELECT dt.IsoYear,
        dt.IsoWkNr as IsoWeek,
        SUM(ISNULL(ca.UrenMeerzorg,0)/7) as HrsMeerzorg
  
FROM    dbo.dimTime dt

        OUTER APPLY
            (
                SELECT TOP 1 M.UrenMeerzorg
         
                FROM dbo.Meerzorg M JOIN HuisAfdeling H ON
                        M.IDHuis = H.IDHuis AND
                        M.IDAfd = H.IDAfd
                        
                WHERE M.Active = 1
                AND    (M.IDHuis = @IDHUIS or @IDHUIS IS NULL)
                AND (H.IDAfd = @IDAFD or @IDAFD IS NULL)        
                AND H.MeeTellen <> 'NEE'
                AND M.[DatumData] <= dt.[Date]
          
                ORDER BY M.[DatumData] DESC
            ) ca

WHERE   IsoYear = @JAAR
        AND (IsoWkNr >= @WEEKBEGIN OR @WEEKBEGIN IS NULL)
        AND (IsoWkNr <= @WEEKEND OR @WEEKEND IS NULL)

GROUP BY IsoYear, IsoWkNr
ORDER BY IsoWkNr