sql學習~with as用法

with ... as 

作用：指定臨時命名的結果集，類似於臨時表。

但是，這個結果集被稱作：公用表表達式(CTE) - common_table_expression

可以在select , insert , update , delete , merge語句的執行范圍定義。

公用表表達式可以包括對自身的引用。所以，這種公用表表達式也稱為遞歸公用表表達式。

語法：

[ WITH <common_table_expression> [ ,...n ] ]  
  
<common_table_expression>::=  
    expression_name [ ( column_name [ ,...n ] ) ]  
    AS  
    ( CTE_query_definition )  

expression_name 

是公用表表達式的有效標識符。即名稱

column_name 

在公用表表達式中指定列名。

當在查詢定義中為所有結果列都提供了不同的名稱時，列名是可選的。即此時可以不寫。

CTE_query_definition

指定一個其結果集填充公用表表達式的 SELECT 語句。即查詢定義。

如果定義了多個 CTE_query_definition，則這些查詢定義必須用下列一個集合運算符聯接起來 ：UNION ALL、UNION、EXCEPT 或 INTERSECT。

 

示例1

顯示每名銷售代表每年的銷售訂單總數。

1.數據准備

create table SalesOrderHeader
(
    SalesPersonID varchar(50),
    SalesOrderID varchar(50),
    OrderDate varchar(50)
)

--drop table SalesOrderHeader
insert into SalesOrderHeader values
('01','20200330001','20200330')
,('02','20200330002','20200330')
,('02','20200331001','20200331')

2.使用

-- Define the CTE expression name and column list.  
WITH Sales_CTE (SalesPersonID, SalesOrderID, SalesYear) AS  
-- Define the CTE query.  
(  
    SELECT SalesPersonID, SalesOrderID, YEAR(OrderDate) AS SalesYear  
    FROM SalesOrderHeader  
    WHERE SalesPersonID IS NOT NULL  
)  
-- Define the outer query referencing the CTE name.  
SELECT SalesPersonID, COUNT(SalesOrderID) AS TotalSales, SalesYear  
FROM Sales_CTE  
GROUP BY SalesYear, SalesPersonID  
ORDER BY SalesPersonID, SalesYear;  

或

省略列名(在查詢中定義了列名)

-- Define the CTE expression name and column list.  
WITH Sales_CTE  
AS  
-- Define the CTE query.  
(  
    SELECT SalesPersonID, SalesOrderID, YEAR(OrderDate) AS SalesYear  
    FROM SalesOrderHeader  
    WHERE SalesPersonID IS NOT NULL  
)  
-- Define the outer query referencing the CTE name.  
SELECT SalesPersonID, COUNT(SalesOrderID) AS TotalSales, SalesYear  
FROM Sales_CTE  
GROUP BY SalesYear, SalesPersonID  
ORDER BY SalesPersonID, SalesYear;  

3.效果

 示例2

如何在單個查詢中定義多個CTE

WITH Sales_CTE (SalesPersonID, TotalSales, SalesYear)  
AS  
-- Define the first CTE query.  
(  
    SELECT SalesPersonID, SUM(TotalDue) AS TotalSales, YEAR(OrderDate) AS SalesYear  
    FROM Sales.SalesOrderHeader  
    WHERE SalesPersonID IS NOT NULL  
       GROUP BY SalesPersonID, YEAR(OrderDate)  
  
)  
,   -- Use a comma to separate multiple CTE definitions.  
  
-- Define the second CTE query, which returns sales quota data by year for each sales person.  
Sales_Quota_CTE (BusinessEntityID, SalesQuota, SalesQuotaYear)  
AS  
(  
       SELECT BusinessEntityID, SUM(SalesQuota)AS SalesQuota, YEAR(QuotaDate) AS SalesQuotaYear  
       FROM Sales.SalesPersonQuotaHistory  
       GROUP BY BusinessEntityID, YEAR(QuotaDate)  
)  
  
-- Define the outer query by referencing columns from both CTEs.  
SELECT SalesPersonID  
  , SalesYear  
  , FORMAT(TotalSales,'C','en-us') AS TotalSales  
  , SalesQuotaYear  
  , FORMAT (SalesQuota,'C','en-us') AS SalesQuota  
  , FORMAT (TotalSales -SalesQuota, 'C','en-us') AS Amt_Above_or_Below_Quota  
FROM Sales_CTE  
JOIN Sales_Quota_CTE ON Sales_Quota_CTE.BusinessEntityID = Sales_CTE.SalesPersonID  
                    AND Sales_CTE.SalesYear = Sales_Quota_CTE.SalesQuotaYear  
ORDER BY SalesPersonID, SalesYear;   

 

示例1和示例2是非遞歸方式，示例3演示遞歸方式

示例3

示例3顯示經理以及向經理報告的雇員的層次列表。

1.數據准備

-- Create an Employee table.  
CREATE TABLE dbo.MyEmployees  
(  
EmployeeID smallint NOT NULL,  
FirstName nvarchar(30)  NOT NULL,  
LastName  nvarchar(40) NOT NULL,  
Title nvarchar(50) NOT NULL,  
DeptID smallint NOT NULL,  
ManagerID int NULL,  
 CONSTRAINT PK_EmployeeID PRIMARY KEY CLUSTERED (EmployeeID ASC)   
);  
-- Populate the table with values.  
INSERT INTO dbo.MyEmployees VALUES   
 (1, N'Ken', N'Sánchez', N'Chief Executive Officer',16,NULL)  
,(273, N'Brian', N'Welcker', N'Vice President of Sales',3,1)  
,(274, N'Stephen', N'Jiang', N'North American Sales Manager',3,273)  
,(275, N'Michael', N'Blythe', N'Sales Representative',3,274)  
,(276, N'Linda', N'Mitchell', N'Sales Representative',3,274)  
,(285, N'Syed', N'Abbas', N'Pacific Sales Manager',3,273)  
,(286, N'Lynn', N'Tsoflias', N'Sales Representative',3,285)  
,(16,  N'David',N'Bradley', N'Marketing Manager', 4, 273)  
,(23,  N'Mary', N'Gibson', N'Marketing Specialist', 4, 16);

2.遞歸使用演示

WITH DirectReports(ManagerID, EmployeeID, Title, EmployeeLevel) AS   
(  
    SELECT ManagerID, EmployeeID, Title, 0 AS EmployeeLevel  
    FROM dbo.MyEmployees   
    WHERE ManagerID IS NULL  
    UNION ALL  
    SELECT e.ManagerID, e.EmployeeID, e.Title, EmployeeLevel + 1  
    FROM dbo.MyEmployees AS e  
        INNER JOIN DirectReports AS d  
        ON e.ManagerID = d.EmployeeID   
)  
SELECT ManagerID, EmployeeID, Title, EmployeeLevel   
FROM DirectReports  
ORDER BY ManagerID;

3.效果

 

示例4

下例演示在單個語句中包含兩個 CTE。 CTE 不能嵌套（無遞歸）

WITH   
 CountDate (TotalCount, TableName) AS  
    (  
     SELECT COUNT(datekey), 'DimDate' FROM DimDate  
    ) ,  
 CountCustomer (TotalAvg, TableName) AS  
    (  
     SELECT COUNT(CustomerKey), 'DimCustomer' FROM DimCustomer  
    )  
SELECT TableName, TotalCount FROM CountDate  
UNION ALL  
SELECT TableName, TotalAvg FROM CountCustomer;

 

參考地址