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  • TSQL with关键字 with as 递归循环表

    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。


    http://www.cnblogs.com/wwan/archive/2011/02/24/1964279.html

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  • 原文地址:https://www.cnblogs.com/y0umer/p/3839030.html
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