Grouping patients and showing repeat visits

CREATE TABLE #tmp (ID INT IDENTITY (1,1), PatientID INT, ActualDate DATETIME);

INSERT INTO #tmp (PatientID, ActualDate)
SELECT 1, '2012-01-20' UNION ALL
SELECT 1, '2012-02-20' UNION ALL
SELECT 1, '2012-03-20' UNION ALL
SELECT 1, '2011-04-20' UNION ALL
SELECT 2, '2012-04-19' UNION ALL
SELECT 2, '2012-04-20' UNION ALL
SELECT 3, '2012-04-19' UNION ALL
SELECT 3, '2011-04-20' UNION ALL
SELECT 4, '2011-04-20';

SELECT PatientID, ActualDate FROM #tmp ORDER BY PatientID, ActualDate;

WITH CTE AS
(
    SELECT PatientID, MAX(ActualDate) AS LastVisit
    FROM #tmp
    GROUP BY PatientID
)
SELECT CTE.PatientID, COUNT(1)-1 AS RepeatVisits
FROM CTE INNER JOIN #tmp ON CTE.PatientID = #tmp.PatientID
WHERE ActualDate BETWEEN DATEADD(month, -6, LastVisit) AND LastVisit
GROUP BY CTE.PatientId;

DROP TABLE #tmp;