SQL INNER JOIN - 连接中的列名也在 select 错误

Question

我对 SQL 还是很陌生，遇到了一些问题。我使用的是经典 Northwind 数据库的变体。我正在使用 Microsoft SQL Server Management Studio。

问题是：

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

这是我得到的：

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

这是我得到的错误：

Msg 8120, Level 16, State 1, Line 2
Column 'Products.productNumber' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

Answer 1

异常文本不言自明。由于您使用的是聚合函数（在您的情况下为 avg），因此您也必须按 Products.productNumber 进行分组。

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

如果要使用聚合函数，必须有group by语句。

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

group by 语句必须包含所有不在聚合函数中的列

Answer 3

如果您使用 AGGREGATE 函数，则必须指定 GROUP BY 子句。

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

Answer 4

如果您使用 AGGREGATE 函数，则必须指定 GROUP BY 子句，此处您选择 ProductNumber 因此在

上指定 GROUP BY 子句

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

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