Script to select specific record from a multiple-records table

CREATE TABLE TestTable ( ID int, TestDate datetime )

GO
INSERT TestTable VALUES(1,'20051231')
INSERT TestTable VALUES(1,'20051230')
INSERT TestTable VALUES(1,'20051230')
INSERT TestTable VALUES(1,'20051229')
INSERT TestTable VALUES(2,'20051230')
INSERT TestTable VALUES(3,'20051229')
INSERT TestTable VALUES(3,'20050101')
GO

The query might be:

SELECT ID,TestDate
FROM TestTable AS tt
WHERE (SELECT COUNT(*)
FROM TestTable AS tc
WHERE tt.ID = tc.ID
AND tt.TestDate < tc.TestDate) = 1
UNION
SELECT ID,MIN(TestDate)
FROM TestTable
GROUP BY ID
HAVING COUNT(ID) = 1
ORDER BY ID