笔记-Microsoft SQL Server 2008技术内幕：T-SQL语言基础-05 表表达式

一般来说，表表达式既不会对性能产生正面影响，也不会对性能产生负面影响。

注意下面这种代码风格：

SELECT orderyear, COUNT(DISTINCT custid) AS numcusts

FROM (SELECT YEAR(orderdate), custid

      FROM Sales.Orders) AS D(orderyear, custid)

GROUP BY orderyear;

公用表表达式

公用表表达式（CTE，Common table expression）是用WITH子句定义的，一般格式为：

WITH USACusts AS

(

  SELECT custid, companyname

  FROM Sales.Customers

  WHERE country = N'USA'

)

SELECT * FROM USACusts;

 和派生表一样，一旦外部查询完成，CTE的生命期就结束了。

CTE分配列别名的方式——内联格式和外部格式，内联格式：

WITH C AS

(

  SELECT YEAR(orderdate) AS orderyear, custid

  FROM Sales.Orders

)

SELECT orderyear, COUNT(DISTINCT custid) AS numcusts

FROM C

GROUP BY orderyear;

 外部格式：

WITH C(orderyear, custid) AS

(

  SELECT YEAR(orderdate), custid

  FROM Sales.Orders

)

SELECT orderyear, COUNT(DISTINCT custid) AS numcusts

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;

 与嵌套的派生表代码相比，上面这种模块化的代码大大提高了代码的可读性和可维护性。

视图

创建一个视图：

USE TSQLFundamentals2008;

IF OBJECT_ID('Sales.USACusts') IS NOT NULL

  DROP VIEW Sales.USACusts;

GO

CREATE VIEW Sales.USACusts

AS



SELECT

  custid, companyname, contactname, contacttitle, address,

  city, region, postalcode, country, phone, fax

FROM Sales.Customers

WHERE country = N'USA';

GO

 记住一点，在定义表表达式的查询语句中不允许出现ORDER BY子句，因为关系表之间的行没有顺序。试图创建一个有序视图也是不合理的，SQL Server将会报错。应该在使用视图的外部查询中使用ORDER BY子句。

内联表值函数

以下代码创建一个内联表值函数：

USE TSQLFundamentals2008;

IF OBJECT_ID('dbo.fn_GetCustOrders') IS NOT NULL

  DROP FUNCTION dbo.fn_GetCustOrders;

GO

CREATE FUNCTION dbo.fn_GetCustOrders

  (@cid AS INT) RETURNS TABLE

AS

RETURN

  SELECT orderid, custid, empid, orderdate, requireddate,

    shippeddate, shipperid, freight, shipname, shipaddress, shipcity,

    shipregion, shippostalcode, shipcountry

  FROM Sales.Orders

  WHERE custid = @cid;

GO

 使用这个函数：

SELECT orderid, custid

FROM dbo.fn_GetCustOrders(1) AS CO;

总结

借助表表达式可以简化代码，提高代码的维护性，还可以封装查询逻辑。当需要使用表表达式时，如果是不计划重用它们的定义，则使用派生表或者CTE；当需要定义可重用的表表达式时，可以使用视图和内联表值函数。