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[数据库]开窗函数 First_Value 和 Last_Value

在Sql server 2012里面,开窗函数丰富了许多,其中带出了2个新的函数 First_Value 和 Last Value .现在来介绍一下这2个函数的应用场景.

首先分析一下First_Value(),用法是根据Partition By对数据进行分区,如果忽略Partition By ,那么默认整块数据一个区域,然后根据Order By 进行排序,取出第一个值。

;WITH CTE AS(SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALLSELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALLSELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALLSELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALLSELECT 5 AS ID ,'2016-06-10' AS DT,'B' AS UName,198 AS TotalAmount )SELECT * ,    FIRST_VALUE(CTE.TotalAmount) OVER (PARTITION BY CTE.UName ORDER BY CTE.ID) AS FirstDeal,    FIRST_VALUE(CTE.DT) OVER (PARTITION BY CTE.UName ORDER BY CTE.ID) AS FirstDate  FROM CTE

ID DT UName TotalAmount FirstDeal FirstDate
----------- ---------- ----- ----------- ----------- ----------
1 2016-06-01 A 135 135 2016-06-01
2 2016-06-05 A 148 135 2016-06-01
3 2016-06-02 B 120 120 2016-06-02
4 2016-06-06 B 153 120 2016-06-02
5 2016-06-10 B 198 120 2016-06-02


在这个场景里面,我求出了根据用户名称(UName)来进行分区,根据ID进行一个排序,求出每个用户第一次购买商品的时间以及交易的金额。如果不使用First_Value 我们也可以换另外一种写法

;WITH CTE AS(SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALLSELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALLSELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALLSELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALLSELECT 5 AS ID ,'2016-06-10' AS DT,'B' AS UName,198 AS TotalAmount )SELECT *   FROM CTE a    CROSS APPLY(SELECT TOP 1 a.TotalAmount AS FirstDeal,DT AS FirstDate FROM CTE WHERE a.UName = CTE.UName ORDER BY CTE.ID) AS b--或者改写成这种形式;WITH CTE AS(SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALLSELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALLSELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALLSELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALLSELECT 5 AS ID ,'2016-06-10' AS DT,'B' AS UName,198 AS TotalAmount )SELECT a.*,b.TotalAmount AS FirstDeal,b.DT AS FirstDate  FROM CTE a    LEFT JOIN CTE b ON a.UName = b.UName AND NOT EXISTS(SELECT * FROM CTE WHERE b.UName = UName AND DT < b.DT)

在这三种写法里面,查询的结果是一致的,燃鹅从查询分析器分析的分析来看,查询性能,使用First_Value 的效率最高,not exists 的效率其次,使用Cross Apply 效率最低。

但是这只是从查询这么少量的测试数据反馈出来的结果,如果具体的场景需要应用,最好是结合实际情况看实际的查询计划来得出最适当的查询效果。

 

然后说下 Last_Value() 的用法,虽然说First_Value 和 Last_Value 一看就想两兄弟。但是!用起来真不是这样的一回事啊!

如果根据First_Value 的解释,那么Last_Value 就是根据Partition进行分区,根据Order By 进行排序返回最后的一个值。想我是这样想的,但是操作起来就不是这么一回事了。

;WITH CTE AS(SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALLSELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALLSELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALLSELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALLSELECT 5 AS ID ,'2016-06-10' AS DT,'B' AS UName,198 AS TotalAmount )SELECT * ,    LAST_VALUE(CTE.TotalAmount) OVER ( PARTITION BY CTE.UName ORDER BY CTE.DT) AS LR  FROM CTEID     DT     UName TotalAmount LR----------- ---------- ----- ----------- -----------1      2016-06-01 A   135     1352      2016-06-05 A   148     1483      2016-06-02 B   120     1204      2016-06-06 B   153     1535      2016-06-10 B   198     198

 

咦!?说好的根据 UName 进行分组,然后再DT进行排序区最后一个价格呢??完全不是这样子啊!!

对,这才是Last_Value的用法,实际上。在我测试的版本里面 (2012,2014), 除了根据 Partition By 进行分区,还对Order by 不一样的值产生不一样的取值。

所以,如果你想看到这个效果,我们不妨把测试样例数据修改一下,把其中2个DT改成一样的,如下面效果

 

;WITH CTE AS(SELECT 1 AS ID ,'2016-06-05' AS DT,'A' AS UName,135 AS TotalAmount UNION ALLSELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALLSELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALLSELECT 4 AS ID ,'2016-06-02' AS DT,'B' AS UName,153 AS TotalAmount UNION ALLSELECT 5 AS ID ,'2016-06-10' AS DT,'B' AS UName,198 AS TotalAmount )SELECT * ,    LAST_VALUE(CTE.TotalAmount) OVER ( PARTITION BY CTE.UName ORDER BY CTE.DT) AS LR  FROM CTEID     DT     UName TotalAmount LR----------- ---------- ----- ----------- -----------1      2016-06-01 A   135     1352      2016-06-05 A   148     1483      2016-06-02 B   120     1204      2016-06-06 B   153     1535      2016-06-10 B   198     198

 

so 现在就看到取值是不一样的,燃鹅,确还是有一个问题,到底哪个才是Last_Value 呢??查询计划说了算~(这个我还真没验证过,请各位大神指导一下)

所以啊,不要看名字就觉得First_Value 和 Last_Value 是亲兄弟啊!!是隔壁老王的啊!!

好,本次分享到这里~