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Select字句在邏輯上是SQL語(yǔ)句最后進(jìn)行處理的最后一步，所以，以下查詢會(huì)發(fā)生錯(cuò)誤：
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- SELECT
- YEAR(OrderDate) AS OrderYear,
- COUNT(DISTINCT CustomerID) AS NumCusts
- FROM dbo.Orders
- GROUP BY OrderYear;
因?yàn)間roup by是在Select之前進(jìn)行的，那個(gè)時(shí)候orderYear這個(gè)列并沒有形成。
如果要查詢成功，可以像下面進(jìn)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;
在作者眼里，他是非常喜歡這種寫法的，因?yàn)楦逦?，更明確，更便于維護(hù)。
在查詢中使用參數(shù)定向產(chǎn)生一批結(jié)果，這個(gè)技巧沒有什么好說的。
嵌套查詢，在處理邏輯上是從里向外進(jìn)行執(zhí)行的。
多重引用，有可能你的SQL語(yǔ)句包含了多次從一個(gè)表進(jìn)行查詢后進(jìn)行連接組合。比如你要比較每年的顧客數(shù)同先前年的顧客數(shù)的變化，所以你的查詢就必須JOIN了2個(gè)相同的表的實(shí)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?yàn)樵跇?biāo)準(zhǔn)的T-SQL語(yǔ)言中已經(jīng)包含了WITH關(guān)鍵字，所以為了區(qū)分，CTE在語(yǔ)句的結(jié)尾加上了“;”作為停止符。
CTE實(shí)例一(結(jié)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(dāng)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(shí)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(shí)例叁(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(shí)例四(修改數(shù)據(jù))
1.把從customer表查詢出來的結(jié)果，動(dòng)態(tài)的組裝進(jìn)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移除數(shù)據(jù)，只保留CustomerDups表里同一CustomerID里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(shí)例五(對(duì)象容器)
即提供了封裝的能力，有利于組件化的編程。作者額外的提醒，CTE無(wú)法直接內(nèi)嵌，但是可以通過把CTE封裝進(jìn)一個(gè)對(duì)象容器里并從一個(gè)外部的CTE里對(duì)這容器的數(shù)據(jù)進(jìn)行查詢而實(shí)現(xiàn)內(nèi)嵌。
作者也說明了，使用CTEs在VIEW和UDFs里是沒有什么價(jià)值的。
有個(gè)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(shí)例六(CTEs的遞歸)
作者給了一個(gè)例子，來講述這個(gè)在SQL2005的新內(nèi)容，CTEs的遞歸。
根據(jù)employeeId，返回此員工的信息，并包含所有下級(jí)員工的信息。(等級(jí)關(guān)系基于empolyeeId和reportsTo的屬性)所返回的結(jié)果包含下列字段，employeeId,reportsTo,FirstName,LastName。
作者在這里，給予了一個(gè)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(gè)索引將通過一個(gè)單獨(dú)的查詢(局部掃描)來取得每個(gè)經(jīng)理的直接下級(jí)。Include(FristName,LastName)加在這里，即是覆蓋列。
小知識(shí)：什么Include索引?
Include索引是SQL2005的新功能。Include索引的列并不影響索引行的物理存儲(chǔ)順序，他們作為一個(gè)掛件‘掛在'索引行上。掛這些‘掛件'的目的在于，只需要掃描一把索引就獲得了這些附加數(shù)據(jù)。
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(gè)遞歸的CTE包含了至少2個(gè)查詢，第一個(gè)查詢?cè)贑TE的身體里類似于一格錨點(diǎn)。這個(gè)錨點(diǎn)僅僅返回一個(gè)有效的表，并作為遞歸的一個(gè)錨。從上的例子看出來，錨點(diǎn)僅僅返回了一個(gè)employeeID = 5 的一行。然后的第2個(gè)查詢是作為遞歸成員。當(dāng)查詢到下屬成員的結(jié)果為空時(shí)，此遞歸結(jié)束。
如果你擔(dān)心遞歸會(huì)造成永久循環(huán)，你可以使用下面的表達(dá)：
- WITH cte_name AS (cte_body) outer_query OPTION (MAXRECURSION n);
默認(rèn)的n為100，當(dāng)n=0時(shí)，無(wú)限制。
原文出處：http://www.cnblogs.com/wwan/archive/2011/02/24/1964279.html
網(wǎng)站欄目：T-SQLwith關(guān)鍵字的實(shí)例應(yīng)用
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