考虑到sql 2005或更高版本：
SELECT T1.X,
T1.Y,
T1.A,
T1.B
FROM
(SELECT X,
Y,
A,
B,
ROW_NUMBER() OVER (Partition BY X,Y Order By A,B) AS RowNum
FROM T
) T1
WHERE T1.RowNum = 1
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