T-SQL with关键字

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实例三(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。

posted @ 2011-02-24 23:16  心的开始  阅读(15372)  评论(8编辑  收藏  举报