SQL Server 2005 T-SQL学习笔记：CTE
时间：2010-08-21 来源：alen88
概念：Common Table Expression，简称CTE，中文可以叫做，通用表表达式．
用处：处理以前版本中SQL不好现实，不好理解，复杂的查询问题．比如：分页，递归查询．．．
基本用法：
WITH <name of your CTE>(<column names>)AS(<actual query>)SELECT * FROM <name of your CTE>
示例一（基本用法）：
with MyCTE(ID, Name)as( select EmployeeID as ID, FirstName + ' ' + LastName as Name from HumanResources.vEmployee)select * from MyCTE
示例二（分页）：
with MyCTE(ID, Name, RowID)as( select EmployeeID as ID, FirstName + ' ' + LastName as Name, Row_Number() over (order by EmployeeID) as RowID from HumanResources.vEmployee)select * from MyCTE where RowID between 1 and 10
示例三（关联CTE）:
with OrderCountCTE(SalesPersonID, OrderCount)as( select SalesPersonID, count(1) from Sales.SalesOrderHeader where SalesPersonID is not null group by SalesPersonID)select sp.SalesPersonID, sp.SalesYTD, cte.OrderCountfrom OrderCountCTE cte inner join Sales.SalesPerson spon cte.SalesPersonID = sp.SalesPersonID order by 3
示例四（使用CTE的删除）：
CREATE TABLE Products ( Product_ID int NOT NULL, Product_Name varchar (25), Price money NULL,CONSTRAINT PK_Products PRIMARY KEY NONCLUSTERED (Product_ID))GOINSERT INTO Products (Product_ID, Product_Name, Price) VALUES (1, 'Widgets', 25)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (2, 'Gadgets', 50)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (3, 'Thingies', 75)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (4, 'Whoozits', 90)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (5, 'Whatzits', 5)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (6, 'Gizmos', 15)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (7, 'Widgets', 24)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (8, 'Gizmos', 36)INSERT INTO Products (Product_ID, Product_Name, Price) VALUES (9, 'Gizmos', 36)GO--==================Delete duplicate products=============================with DuplicateProdCTEas(select Min(Product_ID) as Product_ID, Product_Name from Products group by Product_Name having count(1) >1)delete Products from Products p join DuplicateProdCTE cte on cte.Product_Name = p.Product_Name and p.Product_ID > cte.Product_ID
示例五（递归查询）：
CREATE TABLE Employee_Tree (Employee_NM nvarchar(50), Employee_ID int PRIMARY KEY, ReportsTo int)--insert some data, build a reporting treeINSERT INTO Employee_Tree VALUES('Richard', 1, NULL)INSERT INTO Employee_Tree VALUES('Stephen', 2, 1)INSERT INTO Employee_Tree VALUES('Clemens', 3, 2)INSERT INTO Employee_Tree VALUES('Malek', 4, 2)INSERT INTO Employee_Tree VALUES('Goksin', 5, 4)INSERT INTO Employee_Tree VALUES('Kimberly', 6, 1)INSERT INTO Employee_Tree VALUES('Ramesh', 7, 5)--with MyCTEas( select Employee_ID, Employee_NM, -1 as ReportsTo, 0 as SubLevel from Employee_Tree where ReportsTo is null --root node union all select e.Employee_ID, e.Employee_NM, e.ReportsTo, SubLevel +1 from Employee_Tree e, MyCTE where e.ReportsTo = MyCTE.Employee_ID) --select * from MyCTEselect MyCTE.Employee_NM as emp , MyCTE.SubLevel, e.Employee_NM as boss from MyCTE left join Employee_Tree e on MyCTE.ReportsTo = e.Employee_ID--OPTION(MAXRECURSION 3) --error--OPTION(MAXRECURSION 4) --okwhere SubLevel < 4
注意：OPTION(MAXRECURSION 4)用来设置递归时查找的层数，默认是100，如果超过默认或指定的，则会报错．通常我们用一个层数列来过滤指定的层．
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