開窗函數 First_Value 和 Last_Value

在Sql server 2012裡面，開窗函數豐富了許多，其中帶出了2個新的函數 First_Value 和 Last Value .現在來介紹一下這2個函數的應用場景.

首先分析一下First_Value()，用法是根據Partition By對數據進行分區，如果忽略Partition By ，那麼預設整塊數據一個區域，然後根據Order By 進行排序，取出第一個值。

;WITH CTE AS(
SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALL
SELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALL
SELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALL
SELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALL
SELECT 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

在這個場景裡面，我求出了根據用戶名稱(UName)來進行分區，根據ID進行一個排序，求出每個用戶第一次購買商品的時間以及交易的金額。如果不使用First_Value 我們也可以換另外一種寫法

;WITH CTE AS(
SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALL
SELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALL
SELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALL
SELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALL
SELECT 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

--或者改寫成這種形式

;WITH CTE AS(
SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALL
SELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALL
SELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALL
SELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALL
SELECT 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)

在這三種寫法裡面，查詢的結果是一致的，燃鵝從查詢分析器分析的分析來看，查詢性能，使用First_Value 的效率最高，not exists 的效率其次,使用Cross Apply 效率最低。

但是這隻是從查詢這麼少量的測試數據反饋出來的結果，如果具體的場景需要應用，最好是結合實際情況看實際的查詢計劃來得出最適當的查詢效果。

然後說下 Last_Value() 的用法,雖然說First_Value 和 Last_Value 一看就想兩兄弟。但是！用起來真不是這樣的一回事啊！

如果根據First_Value 的解釋，那麼Last_Value 就是根據Partition進行分區，根據Order By 進行排序返回最後的一個值。想我是這樣想的，但是操作起來就不是這麼一回事了。

;WITH CTE AS(
SELECT 1 AS ID ,'2016-06-01' AS DT,'A' AS UName,135 AS TotalAmount UNION ALL
SELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALL
SELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALL
SELECT 4 AS ID ,'2016-06-06' AS DT,'B' AS UName,153 AS TotalAmount UNION ALL
SELECT 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 CTE


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

咦！？說好的根據 UName 進行分組，然後再DT進行排序區最後一個價格呢？？完全不是這樣子啊！！

對，這才是Last_Value的用法，實際上。在我測試的版本裡面 (2012,2014)， 除了根據 Partition By 進行分區，還對Order by 不一樣的值產生不一樣的取值。

所以，如果你想看到這個效果，我們不妨把測試樣例數據修改一下，把其中2個DT改成一樣的，如下麵效果

;WITH CTE AS(
SELECT 1 AS ID ,'2016-06-05' AS DT,'A' AS UName,135 AS TotalAmount UNION ALL
SELECT 2 AS ID ,'2016-06-05' AS DT,'A' AS UName,148 AS TotalAmount UNION ALL
SELECT 3 AS ID ,'2016-06-02' AS DT,'B' AS UName,120 AS TotalAmount UNION ALL
SELECT 4 AS ID ,'2016-06-02' AS DT,'B' AS UName,153 AS TotalAmount UNION ALL
SELECT 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 CTE



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

so 現在就看到取值是不一樣的，燃鵝，確還是有一個問題，到底哪個才是Last_Value 呢？？查詢計劃說了算~(這個我還真沒驗證過，請各位大神指導一下)

所以啊，不要看名字就覺得First_Value 和 Last_Value 是親兄弟啊！！是隔壁老王的啊！！

好，本次分享到這裡~