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.
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.
表表达式是临时的,只在 SQL 语句的使用期限内有效;表表达式不能共享,但它们比视图更灵活。任何授权的用户都可共享视图定义。