DECLARE @TableA TABLE (ItemID INT, Date DATETIME)
DECLARE @TableB TABLE (ItemID INT, ParentID INT)

INSERT INTO @TableA
SELECT 1, '2012-09-08' UNION ALL SELECT 2, '2012-08-07'
UNION ALL SELECT 3, '2012-09-09' UNION ALL SELECT 4, '2012-09-10'
UNION ALL SELECT 5, '2012-08-23'

INSERT INTO @TableB
SELECT 1, 2 UNION ALL SELECT 3, 2

;WITH CTE AS (
    SELECT b.ItemID, b.ParentID, a.Date
    FROM @TableB b
    INNER JOIN @TableA a ON b.ParentID = a.ItemID
    UNION ALL
    SELECT a.ItemID, NULL, a.Date
    FROM @TableA a
    )
SELECT a.ItemID, b.Date
FROM (
    SELECT ItemID, Date, ParentID
        ,rn=ROW_NUMBER() OVER (PARTITION BY ItemID ORDER BY ParentID DESC)
    FROM CTE
    ) a
INNER JOIN @TableA b ON a.ItemID = b.ItemID 
WHERE rn=1
ORDER BY a.Date, ParentID, ItemID