SQL INNER JOIN - 連接中的列名也在選擇*錯誤*

Question

我對SQL仍然很陌生，遇到問題。我正在使用經典Northwind數據庫的變體。我正在使用Microsoft SQL Server Management Studio。

這裏的問題是：

/* 
Join Products and ProductVendors to display by product, the product 
number, the average wholesale price, and the average retail price. 
*/ 

下面是我有：

SELECT Products.productNumber As [Product Number], 
     CAST(AVG(wholesalePrice) as decimal (8,2)) As [Average Wholesale Price], 
     CAST(AVG(retailPrice) as decimal (8,2)) As [Average Retail Price] 
FROM Products INNER JOIN ProductVendors 
     ON Products.productNumber = ProductVendors.productNumber 

這是我得到的錯誤：

消息8120 ，等級16，狀態1，行2
列'Products.productNumber'在選擇列表中無效，因爲它不包含在聚合函數或GROUP BY子句中。

Answer 1

異常文本是不言自明的。既然你正在使用聚合函數（avg你的情況），你也必須由Products.productNumber進行分組。

select Products.productNumber aa [Product Number], 
     CAST(AVG(wholesalePrice) as decimal (8,2)) as [Average Wholesale Price], 
     CAST(AVG(retailPrice) as decimal (8,2)) as [Average Retail Price] 
from Products inner join ProductVendors 
    on Products.productNumber = ProductVendors.productNumber 
group by Products.productNumber 

Answer 2

如果要使用聚合函數，則必須具有group by語句。

SELECT Products.productNumber As [Product Number], 
     CAST(AVG(wholesalePrice) as decimal (8,2)) As [Average Wholesale Price], 
     CAST(AVG(retailPrice) as decimal (8,2)) As [Average Retail Price] 
FROM Products INNER JOIN ProductVendors ON Products.productNumber = ProductVendors.productNumber 
group by Products.productNumber 

一個的group by聲明必須包含所有列至極不是一個聚合函數

Answer 3

如果使用AGGREGATE功能則必須指定GROUP BY條款。

SELECT Products.productNumber As [Product Number], 
     CAST(AVG(wholesalePrice) as decimal (8,2)) As [Average Wholesale Price], 
     CAST(AVG(retailPrice) as decimal (8,2)) As [Average Retail Price] 
FROM Products 
INNER JOIN ProductVendors ON Products.productNumber = ProductVendors.productNumber 
GROUP BY Products.productNumber 

Answer 4

如果使用AGGREGATE功能則GROUP BY條款，必須在這裏指定您選擇ProductNumber使指定對

SELECT Products.productNumber As [Product Number], 
     CAST(AVG(wholesalePrice) as decimal (8,2)) As [Average Wholesale Price], 
     CAST(AVG(retailPrice) as decimal (8,2)) As [Average Retail Price] 
FROM Products 
INNER JOIN ProductVendors ON Products.productNumber = ProductVendors.productNumber 
GROUP BY Products.productNumber