cte的用法

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．




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