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;