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