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 程式師世界 >> 數據庫知識 >> SqlServer數據庫 >> 關於SqlServer >> SqlServer 2005 T-SQL Query 學習筆記(1)

SqlServer 2005 T-SQL Query 學習筆記(1)

編輯:關於SqlServer

Select字句在邏輯上是SQL語句最後進行處理的最後一步,所以,以下查詢會發生錯誤:

SELECT
 YEAR(OrderDate) AS OrderYear,
 COUNT(DISTINCT CustomerID) AS NumCusts
FROM dbo.Orders
GROUP BY OrderYear;

因為group by是在Select之前進行的,那個時候orderYear這個列並沒有形成。

如果要查詢成功,可以像下面進行修改:

SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
FROM (SELECT YEAR(OrderDate) AS OrderYear, CustomerID
   FROM dbo.Orders) AS D
GROUP BY OrderYear;

還有一種很特殊的寫法:

SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
FROM (SELECT YEAR(OrderDate), CustomerID
   FROM dbo.Orders) AS D(OrderYear, CustomerID)
GROUP BY OrderYear;

在作者眼裡,他是非常喜歡這種寫法的,因為更清晰,更明確,更便於維護。

在查詢中使用參數定向產生一批結果,這個技巧沒有什麼好說的。

嵌套查詢,在處理邏輯上是從裡向外進行執行的。

多重引用,有可能你的SQL語句包含了多次從一個表進行查詢後進行連接組合。比如你要比較每年的顧客數同先前年的顧客數的變化,所以你的查詢就必須JOIN了2個相同的表的實例,這也是不可避免的。

Common Table Expressions (CTE)

CTE是在SQL2005新加入的一種表的表示類型。

它的定義如下:

WITH cte_name

AS

(

cte_query

)

outer_query_refferring to_cte_name;

注意:因為在標准的T-SQL語言中已經包含了WITH關鍵字,所以為了區分,CTE在語句的結尾加上了“;”作為停止符。

CTE實例一(結果集別名)

WITH C AS
(
 SELECT YEAR(OrderDate) AS OrderYear, CustomerID
 FROM dbo.Orders
)
SELECT OrderYear, COUNT(DISTINCT CustomerID) AS NumCusts
FROM C
GROUP BY OrderYear;

當然,作者本人有更推薦的寫法:

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實例二(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實例三(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實例四(修改數據)

1.把從customer表查詢出來的結果,動態的組裝進新表CustomersDups裡:

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移除數據,只保留CustomerDups表裡同一CustomerID裡KeyCol為最大的記錄。

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實例五(對象容器)

即提供了封裝的能力,有利於組件化的編程。作者額外的提醒,CTE無法直接內嵌,但是可以通過把CTE封裝進一個對象容器裡並從一個外部的CTE裡對這容器的數據進行查詢而實現內嵌。

作者也說明了,使用CTEs在VIEW和UDFs裡是沒有什麼價值的。

有個例子,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實例六(CTEs的遞歸)

作者給了一個例子,來講述這個在SQL2005的新內容,CTEs的遞歸。

根據employeeId,返回此員工的信息,並包含所有下級員工的信息。(等級關系基於empolyeeId和reportsTo的屬性)所返回的結果包含下列字段,employeeId,reportsTo,FirstName,LastName。

作者在這裡,給予了一個最佳的索引方式:

CREATE UNIQUE INDEX idx_mgr_emp_ifname_ilname
 ON dbo.Employees(ReportsTo, EmployeeID)
 INCLUDE(FirstName, LastName);

作者的解釋: 這個索引將通過一個單獨的查詢(局部掃描)來取得每個經理的直接下級。Include(FristName,LastName)加在這裡,即是覆蓋列。

小知識:什麼Include索引?

Include索引是SQL2005的新功能。Include索引的列並不影響索引行的物理存儲順序,他們作為一個掛件‘掛在'索引行上。掛這些‘掛件'的目的在於,只需要掃描一把索引就獲得了這些附加數據。

回到作者的例子上,下面是遞歸的代碼:

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:一個遞歸的CTE包含了至少2個查詢,第一個查詢在CTE的身體裡類似於一格錨點。這個錨點僅僅返回一個有效的表,並作為遞歸的一個錨。從上的例子看出來,錨點僅僅返回了一個employeeID = 5 的一行。然後的第2個查詢是作為遞歸成員。當查詢到下屬成員的結果為空時,此遞歸結束。

如果你擔心遞歸會造成永久循環,你可以使用下面的表達:

WITH cte_name AS (cte_body) outer_query OPTION (MAXRECURSION n);

默認的n為100,當n=0時,無限制。

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