DECLARE @NumOfVisits TABLE
(
   CustomerID INT NOT NULL,
   MonthID INT NOT NULL,
   MonthDate DATE NOT NULL,
   NumOfVisits INT NOT NULL,
   PRIMARY KEY(CustomerID, MonthDate)
);

INSERT INTO @NumOfVisits(CustomerID, MonthID, MonthDate, NumOfVisits)
   VALUES(001, 200405, '20040501', 1),
         (001, 200406, '20040601', 2),
         (001, 200407, '20040701', 1),
         (001, 200505, '20050501', 5),
         (001, 200507, '20050701', 10),
         (002, 200811, '20081101', 3),
         (002, 200811, '20081120', 1);

SELECT T1.CustomerID, T1.MonthID, T2.NumOfVisitsInLast12Month
  FROM (SELECT T1.CustomerID, T1.MonthID,
               DATEADD(DAY, -1, DATEADD(MONTH, 1, CAST(CAST(T1.MonthID AS CHAR(6)) + '01' AS DATE))) AS MonthDate
          FROM @NumOfVisits AS T1
         GROUP BY T1.CustomerID, T1.MonthID) AS T1
       CROSS APPLY
       (SELECT SUM(T2.NumOfVisits) AS NumOfVisitsInLast12Month
          FROM @NumOfVisits AS T2
         WHERE T2.CustomerID = T1.CustomerID
           AND T2.MonthDate <= T1.MonthDate
           AND T2.MonthDate > DATEADD(MONTH, -13, T1.MonthDate)) AS T2;