最近在研究 T-SQL 遞迴的作法

還是"看無"

 

 


 

 

寂靜的寂寞…

 

概念:Common Table Expression,簡稱CTE,中文可以叫做,通用表表達式.

用處:處理以前版本中SQL不好現實,不好理解,複雜的查詢問題.比如:分頁,遞歸查詢...

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(分頁):

 

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(關聯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的刪除):

 

 

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(遞歸查詢):

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)用來設置遞歸時查找的層數,默認是100,如果超過默認或指定的,則會報錯.通常我們用一個層數列來過濾指定的層.

 

 

創作者介紹

我只是個打雜小弟 發表在 痞客邦 留言(0) 人氣()