Select字句在逻辑上是SQL语句最后进行处理的最后一步，所以，以下查询会发生错误：
- SELECT
- YEAR(OrderDate) AS OrderYear,
- COUNT(DISTINCT CustomerID) AS NumCusts
- FROM dbo.Orders
- GROUP BY OrderYear;
因为group by是在Select之前进行的，那个时候orderYear这个列并没有形成。
如果要查询成功，可以像下面进行修改：
- SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
- FROM (SELECT YEAR(OrderDate) AS OrderYear, CustomerID
- FROM dbo.Orders) AS D
- GROUP BY OrderYear;
还有一种很特殊的写法：
- SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
- FROM (SELECT YEAR(OrderDate), CustomerID
- FROM dbo.Orders) AS D(OrderYear, CustomerID)
- GROUP BY OrderYear;
在作者眼里，他是非常喜欢这种写法的，因为更清晰，更明确，更便于维护。
在查询中使用参数定向产生一批结果，这个技巧没有什么好说的。
嵌套查询，在处理逻辑上是从里向外进行执行的。
多重引用，有可能你的SQL语句包含了多次从一个表进行查询后进行连接组合。比如你要比较每年的顾客数同先前年的顾客数的变化，所以你的查询就必须JOIN了2个相同的表的实例，这也是不可避免的。
Common Table Expressions (CTE)
CTE是在SQL2005新加入的一种表的表示类型。
它的定义如下：
- WITH cte_name
- AS
- (
- cte_query
- )
- outer_query_refferring to_cte_name;
注意：因为在标准的T-SQL语言中已经包含了WITH关键字，所以为了区分，CTE在语句的结尾加上了“;”作为停止符。
CTE实例一(结果集别名)
- WITH C AS
- (
- SELECT YEAR(OrderDate) AS OrderYear, CustomerID
- FROM dbo.Orders
- )
- SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
- FROM C
- GROUP BY OrderYear;
当然，作者本人有更推荐的写法：
- WITH C(OrderYear, CustomerID) AS
- (
- SELECT YEAR(OrderDate), CustomerID
- FROM dbo.Orders
- )
- SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
- FROM C
- GROUP BY OrderYear;
CTE实例二(多重CTEs)
- WITH C1 AS
- (
- SELECT YEAR(OrderDate) AS OrderYear, CustomerID
- FROM dbo.Orders
- ),
- C2 AS
- (
- SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
- FROM C1
- GROUP BY OrderYear
- )
- SELECT OrderYear, NumCusts
- FROM C2
- WHERE NumCusts > 70;
CTE实例叁(多重引用)
- WITH YearlyCount AS
- (
- SELECT YEAR(OrderDate) AS OrderYear,
- COUNT(DISTINCT CustomerID) AS NumCusts
- FROM dbo.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 OUTER JOIN YearlyCount AS Prv
- ON Cur.OrderYear = Prv.OrderYear + 1;
CTE实例四(修改数据)
1.把从customer表查询出来的结果，动态的组装进新表CustomersDups里：
- IF OBJECT_ID('dbo.CustomersDups') IS NOT NULL
- DROP TABLE dbo.CustomersDups;
- GO
- WITH CrossCustomers AS
- (
- SELECT 1 AS c, C1.*
- FROM dbo.Customers AS C1, dbo.Customers AS C2
- )
- SELECT ROW_NUMBER() OVER(ORDER BY c) AS KeyCol,
- CustomerID, CompanyName, ContactName, ContactTitle, Address,
- City, Region, PostalCode, Country, Phone, Fax
- INTO dbo.CustomersDups
- FROM CrossCustomers;
2.使用CTE移除数据，只保留CustomerDups表里同一CustomerID里KeyCol为最大的记录。
- WITH JustDups AS
- (
- SELECT * FROM dbo.CustomersDups AS C1
- WHERE KeyCol <
- (SELECT MAX(KeyCol) FROM dbo.CustomersDups AS C2
- WHERE C2.CustomerID = C1.CustomerID)
- )
- DELETE FROM JustDups;
CTE实例五(对象容器)
即提供了封装的能力，有利于组件化的编程。作者额外的提醒，CTE无法直接内嵌，但是可以通过把CTE封装进一个对象容器里并从一个外部的CTE里对这容器的数据进行查询而实现内嵌。
作者也说明了，使用CTEs在VIEW和UDFs里是没有什么价值的。
有个例子，如下:
- CREATE VIEW dbo.VYearCnt
- AS
- WITH YearCnt AS
- (
- SELECT YEAR(OrderDate) AS OrderYear,
- COUNT(DISTINCT CustomerID) AS NumCusts
- FROM dbo.Orders
- GROUP BY YEAR(OrderDate)
- )
- SELECT * FROM YearCnt;
CTE实例六(CTEs的递归)
作者给了一个例子，来讲述这个在SQL2005的新内容，CTEs的递归。
根据employeeId，返回此员工的信息，并包含所有下级员工的信息。(等级关系基于empolyeeId和reportsTo的属性)所返回的结果包含下列字段，employeeId,reportsTo,FirstName,LastName。
作者在这里，给予了一个最佳的索引方式：
- CREATE UNIQUE INDEX idx_mgr_emp_ifname_ilname
- ON dbo.Employees(ReportsTo, EmployeeID)
- INCLUDE(FirstName, LastName);
作者的解释： 这个索引将通过一个单独的查询(局部扫描)来取得每个经理的直接下级。Include(FristName,LastName)加在这里，即是覆盖列。
小知识：什么Include索引?
Include索引是SQL2005的新功能。Include索引的列并不影响索引行的物理存储顺序，他们作为一个挂件‘挂在'索引行上。挂这些‘挂件'的目的在于，只需要扫描一把索引就获得了这些附加数据。
回到作者的例子上，下面是递归的代码：
- WITH EmpsCTE AS
- (
- SELECT EmployeeID, ReportsTo, FirstName, LastName
- FROM dbo.Employees
- WHERE EmployeeID = 5
- UNION ALL
- SELECT EMP.EmployeeID, EMP.ReportsTo, EMP.FirstName, EMP.LastName
- FROM EmpsCTE AS MGR
- JOIN dbo.Employees AS EMP
- ON EMP.ReportsTo = MGR.EmployeeID
- )
- SELECT * FROM EmpsCTE;
理解：一个递归的CTE包含了至少2个查询，第一个查询在CTE的身体里类似于一格锚点。这个锚点仅仅返回一个有效的表，并作为递归的一个锚。从上的例子看出来，锚点仅仅返回了一个employeeID = 5 的一行。然后的第2个查询是作为递归成员。当查询到下属成员的结果为空时，此递归结束。
如果你担心递归会造成永久循环，你可以使用下面的表达：
- WITH cte_name AS (cte_body) outer_query OPTION (MAXRECURSION n);
默认的n为100，当n=0时，无限制。
原文出处：http://www.cnblogs.com/wwan/archive/2011/02/24/1964279.html
