Group by questo sconosciuto

select CustomerID, OrderDate 
from dbo.Orders
where CustomerID='ALFKI'

CustomerID         OrderDate 
ALFKI                1997-10-03 00:00:00.000
ALFKI                1997-10-13 00:00:00.000

CustomerID       OrderDate   
ALFKI              03,13 Ottobre

select 
  CustomerID
  , year(OrderDate) as anno
  , month(OrderDate) as mese
  , concatenamiGiorni(OrderDate) as giorni
from dbo.Orders
where CustomerID='ALFKI'
group by CustomerID, year(OrderDate), month(OrderDate)

>Prova a vedere questo post se può esserti di aiuto

SELECT Durata, dbo.ufn_Concat(Durata)
  FROM dbo.Dati
  GROUP BY Durata; 

select 
  CustomerID
  , year(OrderDate) as anno
  , month(OrderDate) as mese
  , dbo.ufn_Concat(OrderDate) as giorni
from dbo.Orders
where CustomerID='ALFKI'
group by CustomerID, year(OrderDate), month(OrderDate)

USE tempdb;

CREATE TABLE dbo.Orders(
CustomerID char(5) NOT NULL,
OrderDate datetime NOT NULL
);

INSERT dbo.Orders VALUES('ALFKI', '19970101 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971003 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971013 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971101 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971102 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971130 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971201 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971205 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19971231 00:00:00.000');
INSERT dbo.Orders VALUES('ALFKI', '19980101 00:00:00.000');
GO

CREATE FUNCTION dbo.ufn_GetLastMonthDay(
@Date datetime
)
RETURNS datetime
BEGIN
    RETURN DATEADD(month, DATEDIFF(month, 0, @Date) + 1, 0) - 1;
END
GO

CREATE FUNCTION dbo.ufn_ConcatenaGiorni(
@CustomerID char(5),
@Anno int,
@Mese int
)
RETURNS varchar(122) 
BEGIN 
    DECLARE @Output varchar(122);
    SET @Output = '';

    /* Calcolo il primo del mese */
    DECLARE @OrderDate datetime;
    SET @OrderDate = DATEADD(year, @Anno - 1900, 0) + DATEADD(month, @Mese - 1, 0);

    SELECT @Output = @Output + ', ' + CONVERT(char(2), OrderDate, 103) 
    FROM dbo.Orders 
    WHERE CustomerID = @CustomerID
    AND OrderDate BETWEEN @OrderDate AND dbo.ufn_GetLastMonthDay(@OrderDate)
    ORDER BY OrderDate;

    RETURN STUFF(@Output, 1, 2, '');
END 
GO

SELECT
      Q.*
    , dbo.ufn_ConcatenaGiorni(Q.CustomerID, Q.Anno, Q.Mese) AS Giorni
FROM (    
    SELECT 
          CustomerID
        , YEAR(OrderDate) AS Anno
        , MONTH(OrderDate) AS Mese
    FROM dbo.Orders
    WHERE CustomerID = 'ALFKI'
    GROUP BY CustomerID, YEAR(OrderDate), MONTH(OrderDate)
) AS Q;

/* Output:

CustomerID Anno        Mese        Giorni
---------- ----------- ----------- -----------
ALFKI      1997        1           01
ALFKI      1997        10          03, 13
ALFKI      1997        11          01, 02, 30
ALFKI      1997        12          01, 05, 31
ALFKI      1998        1           01

(5 row(s) affected)

*/

DROP FUNCTION dbo.ufn_ConcatenaGiorni, dbo.ufn_GetLastMonthDay;
DROP TABLE dbo.Orders;