Zapytanie SQL Server - oblicz dostępność pokoi

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DECLARE @Reservation TABLE ( BookDate DATE, ROOMS INT)
 INSERT INTO @Reservation VALUES 
('1-Jul-2015',30 ), 
('2-Jul-2015',30 ),
('3-Jul-2015',30 ),
('5-Jul-2015',28 ),
('6-Jul-2015',28 ),
('7-Jul-2015',28 ),
('8-Jul-2015',30 ),
('9-Jul-2015',30 ),
('10-Jul-2015',26 ),
('11-Jul-2015',28 ),
('12-Jul-2015',28 )


    ;WITH
    cte AS (
        select ROW_NUMBER() OVER(ORDER BY BookDate) AS RowNumber,
        [ROOMS], BookDate FROM @Reservation
    ),
    cte2 as (
        SELECT TOP 1 RowNumber, 1 as GroupNumber, [ROOMS], BookDate FROM cte ORDER BY RowNumber
        UNION ALL
        SELECT c1.RowNumber,
            CASE WHEN c2.[ROOMS] <> c1.[ROOMS] then c2.GroupNumber + 1 ELSE c2.GroupNumber END AS  GroupNumber, c1.[ROOMS], c1.BookDate 
        FROM cte2 c2 join cte c1 on c1.RowNumber = c2.RowNumber + 1     
    )
    SELECT Start_Date, End_Date, Rooms
    FROM
    (   SELECT MIN(BookDate) AS START_DATE, MAX(BookDate) AS END_DATE ,ROOMS, GroupNumber
        FROM cte2
        GROUP BY  ROOMS ,GroupNumber
    ) a

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