SELECT     YEAR(m1.dateofposting)  AS YEAR,
           MONTH(m1.dateofposting) AS MONTH,
           COUNT(m1.message)       AS policzofmessage
FROM       Messages m1
INNER JOIN
           (
                    SELECT   m3 .username,
                             YEAR(m3 .dateofposting)  AS YEAR,
                             MONTH(m3 .dateofposting) AS MONTH,
                             COUNT(m3 .message)       AS policzofmessage
                    FROM     Messages m3
                    GROUP BY m3 .username,
                             m3.YEAR,
                             m3.MONTH
           ) m2
ON         m1.YEAR=m2.YEAR,
           m1.MONTH=m2.MONTH,
           m1.policzofmessage=m2.policzofmessage
GROUP BY   YEAR(m1.[DateOfPosting]),
           MONTH(m1.[DateOfPosting])