SQL Server Compact Edition 3.5的SQL查询 - GROUP BY问题

Question

    SELECT BabyInformation.* , t1.* 
    FROM   BabyInformation 
    LEFT  JOIN
      (SELECT *  FROM  BabyData 
       GROUP BY BabyID 
       ORDER By Date DESC  ) AS t1 ON BabyInformation.BabyID=t1.BabyID

这是我的查询。我希望根据日期获得最新的BabyData元组。 BabyInformation应该与babyData联系，但每个宝宝只有一行......

我试过TOP(1)，但这只适用于第一个孩子

Answer 1

这是一种方法，还有其他方法可以更快，但我相信这对初学者来说是最清楚的。

 SELECT BabyInformation.*, BabyData.*
    FROM   BabyInformation 
    JOIN
      (SELECT BabyID, Max(Date) as maxDate  FROM  BabyData 
       GROUP BY BabyID 
      ) AS t1 
 ON BabyInformation.BabyID=t1.BabyID 
    Join BabyData ON BabyData.BabyID = t1.BabyID and BabyData.Date = t1.maxDate

Answer 2

这应该这样做：

SELECT bi.* , bd.* 
    FROM BabyInformation [bi]
    LEFT JOIN BabyData [bd]
        on bd.BabyDataId = (select top 1 sub.BabyDataId from BabyData [sub] where sub.BabyId = bi.BabyId order by sub.Date desc)

我假设BabyData表中有一个名为'BabyDataId'的列。