ALTER function [dbo].[GetGGpresenze] (@data1 date,@Date2 date)
Returns Int
WITH EXECUTE AS CALLER
AS
BEGIN
DECLARE @Giorni Int
SET @giorni =Case DATEDIFF(day,@data1,@Date2) when '0' then '1' else DATEDIFF(day,@Data1,@Date2)End  
SET @giorni=Isnull(@giorni,0)
RETURN(@giorni)
END

Select count(m.arrivo) totarrivi, count(m.partenza) totpartiti,sum(dbo.GetGGpresenze (m.arrivo,case when m.partenza is null then DATEADD(day,1,@data2) else m.partenza end)) totpresenze
From Tab_movimenti m
Where
m.Idstruttura=@idstruttura
and
m.arrivo between @data1 and @data2