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  • 公用表表达式CTE

      公用表表达式CTE表面上和派生表非常相似,看起来只是语义上的区别。但和派生表比较起来,CTE具有几个优势:第一,如果须要在一个CTE中引用另一个CTE,不需要像派生表那样嵌套,相反,只要简单地在同一个WITH子句中定义多个CTE,并用逗号把它们分隔开。每个CTE可以引用在它前面定义的所有CTE。而外部查询可以引用所有CTE。

      下面是一个公用表表达式CTE的示例。

    USE TSQLFundamentals2008;
    GO
    
    -- 公用表表达式CTE
    -- 一个简单的公用表表达式的例子
    WITH USACusts AS 
    (
        SELECT custid,companyname FROM Sales.Customers
        WHERE country= N'USA'
    )
    SELECT * FROM USACusts;
    
    -- 分配列别名
    WITH C AS 
    (
        SELECT YEAR(orderdate) AS orderyear,custid FROM Sales.Orders
    )
    SELECT orderyear, COUNT(DISTINCT custid) FROM C
    GROUP BY orderyear;
    
    -- 使用参数
    DECLARE @empid AS INT=3;
    
    WITH C AS 
    (
        SELECT YEAR(orderdate) AS orderyear,custid FROM Sales.Orders
        WHERE empid= @empid
    )
    SELECT orderyear, COUNT(DISTINCT custid) 
    FROM C
    GROUP BY orderyear;
    
    -- 定义多个CTE
    WITH C1 AS 
    (
        SELECT YEAR(orderdate) AS orderyear, custid FROM Sales.Orders
    ),
    C2 AS 
    (
        SELECT orderyear,COUNT(DISTINCT custid) AS numcusts FROM C1 
        GROUP BY orderyear
    )
    SELECT orderyear,numcusts FROM C2
    WHERE numcusts>70;
    
    -- CTE的多引用
    WITH YearlyCount AS 
    (
        SELECT YEAR(orderdate) AS orderyear, COUNT(DISTINCT custid) AS numcusts FROM Sales.Orders
        GROUP BY YEAR(orderdate)
    )
    SELECT Cur.orderyear, Cur.numcusts AS curnumcusts,Prv.numcusts AS prvnumcusts,Cur.numcusts-Prv.numcusts AS growth
    FROM YearlyCount AS Cur
    LEFT JOIN YearlyCount AS Prv ON Cur.orderyear = Prv.orderyear+1;
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  • 原文地址:https://www.cnblogs.com/mcgrady/p/3993349.html
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