派生表

　　從這一篇開始要介紹表表達式了，什么是表表達式呢？表表達式是一種命名的查詢表達式，代表一個有效的關系表。可以像其它表一樣，在數據處理語句中使用表表達式。SQL Server支持四種類型的表表達式：派生表，公用表表達式，視圖和內聯表值函數。

　　派生表(也稱為表子查詢)是在外部查詢的FROM子句中定義的。派生表的存在范圍為定義它的外部查詢，只要外部查詢一結束，派生表就不存在了。定義派生表的查詢語句要寫在一對圓括號內，后面跟着AS子句和派生表的名稱。

　　下面是派生表的示例代碼。

USE TSQLFundamentals2008;
GO

-- 派生表
-- 一個簡單的派生表示例
SELECT * 
FROM (SELECT custid,companyname FROM Sales.Customers WHERE country= N'USA') AS USACusts;

-- 原來寫法
SELECT YEAR(orderdate) AS orderyear, COUNT(DISTINCT custid) AS numcusts 
FROM Sales.Orders
GROUP BY YEAR(orderdate);

-- 分配列別名
SELECT orderyear,COUNT(DISTINCT custid) AS numcusts 
FROM (SELECT YEAR(orderdate) AS orderyear,custid FROM Sales.Orders) AS Orders1
GROUP BY orderyear;

-- 使用參數
DECLARE @empid AS INT=3;

SELECT orderyear,COUNT(DISTINCT custid) AS numcusts 
FROM (SELECT YEAR(orderdate) AS orderyear,custid FROM Sales.Orders WHERE empid= @empid) AS Orders1
GROUP BY orderyear;

-- 嵌套
SELECT orderyear, numcusts 
FROM (SELECT orderyear, COUNT(DISTINCT custid) AS numcusts 
        FROM (SELECT YEAR(orderdate) AS orderyear,custid FROM Sales.Orders) AS Orders1
        GROUP BY orderyear) AS Orders2
WHERE numcusts>70;