MySQL每个分类的前几条记录

SELECT A1.*
FROM Article AS A1
    INNER JOIN (SELECT A.Category,A.InsertDate
        FROM Article AS A
        LEFT JOIN Article AS B
        ON A.Category = B.Category
        AND A.InsertDate <= B.InsertDate
        GROUP BY A.Category,A.InsertDate
        HAVING COUNT(B.InsertDate) <= @N
    ) AS B1
ON A1.Category = B1.Category
AND A1.InsertDate = B1.InsertDate
ORDER BY A1.Category,A1.InsertDate DESC