CTE 通用表表达式

CTE 通用表表达式

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.OrderCount from OrderCountCTE cte inner join Sales.SalesPerson sp on 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) ) GO INSERT 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 DuplicateProdCTE as ( 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 tree INSERT 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 MyCTE as ( 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 MyCTE select 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) --ok where 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．