使用 GROUP BY 从多个表中选择

Posted 2023-05-07

【中文标题】使用 GROUP BY 从多个表中选择

【英文标题】：SELECT from multiple tables with GROUP BY

【发布时间】：2012-09-16 13:49:40

【问题描述】：

我在多表查询方面遇到了一点问题。 （RDBMS：访问）

这是数据库模式：（此查询中仅使用 S_Cards、Books、Authors、Students 表） S_Cards 是学生书单（在图书馆）。

查询： 选择学生中最受欢迎的作者以及该作者在图书馆订购的书籍的数量。

虽然我可以在一个查询中得到这样的订单+作者列表：

SELECT 
    Students.FirstName & " " & Students.LastName AS [Student], 
    Books.Name AS [Book], Authors.FirstName & " " & Authors.LastName AS [Author]
FROM 
    Students, 
    Books, 
    S_Cards, 
    Authors
WHERE 
    S_Cards.ID_Student = Students.ID 
AND S_Cards.ID_Book = Books.ID 
AND Books.ID_Author = Authors.ID
ORDER BY Authors.LastName

结果（抱歉，是俄语）：

我不明白，为什么我不能像这样 COUNT 和 GROUP BY：

SELECT 
    Students.FirstName & " " & Students.LastName AS [Student], 
    Books.Name AS [Book], 
    COUNT(Authors.FirstName & " " & Authors.LastName) AS [Number of books]
FROM Students, Books, S_Cards, Authors
WHERE 
    S_Cards.ID_Student = Students.ID 
AND S_Cards.ID_Book = Books.ID 
AND Books.ID_Author = Authors.ID
GROUP BY 3

我收到一条错误消息，“Authors.FirstName & " " & Authors.LastName' 不是静态函数或组的一部分。

问题：

有没有办法在没有 JOIN 的情况下执行此查询，仅通过 GROUP BY、SELECT、UNION 以及如何执行？ 我的第二个查询有什么问题？

【参考方案1】：

解决方案（摘自older revision in question）：

SELECT TOP 1 Author, COUNT(Book) AS [Number of books] FROM
(
    SELECT 
        Students.FirstName & " " & Students.LastName AS [Student], 
        Books.Name AS [Book], 
        Authors.FirstName & " " & Authors.LastName AS [Author]
    FROM 
        Students, 
        Books, 
        S_Cards, 
        Authors
    WHERE 
        S_Cards.ID_Student = Students.ID AND
        S_Cards.ID_Book = Books.ID AND
        Books.ID_Author = Authors.ID
    ORDER BY Authors.LastName
)
GROUP BY Author
ORDER BY 2 DESC

【参考方案2】：

SQL Server 中的字符串concatenation operator 是+ 而不是&。此外，您应该按非聚合函数的事物进行分组。

SELECT Students.FirstName + ' ' + Students.LastName AS [Student]
     , Books.Name AS [Book]
     , COUNT(Authors.FirstName + ' ' + Authors.LastName) AS [Number of books]
  FROM Students
  JOIN S_Cards
    ON S_Cards.ID_Student = Students.ID
  JOIN BOOKS
    ON S_Cards.ID_Book = Books.ID 
  JOIN Authors
    ON Books.ID_Author = Authors.ID
 GROUP BY Students.FirstName + ' ' + Students.LastName
        , Books.Name

请注意，我已将您的查询更改为标准的 ANSI 连接语法，这使得错误更难产生并且更易于阅读。

仔细想想，你的计数似乎有点奇怪。书的数量不是COUNT(Books.ID)吗？

【讨论】：

这假设您正在使用 SQL Server（看起来很像）。以后请始终使用适当的 RDBMS 标记问题。

感谢您的回复。我认为 COUNT(Books.ID) 会计算订购某本书的次数。我需要计算作者的任何书被订购了多少次，而不是特定的书。 RDBMS 是 Access。

【参考方案3】：

您必须对不属于聚合函数的任何内容进行 GROUP BY：

SELECT 
    Students.FirstName & " " & Students.LastName AS [Student], 
    Books.Name AS [Book], 
    COUNT(Authors.FirstName & " " & Authors.LastName) AS [Number of books]
FROM Students, Books, S_Cards, Authors
WHERE 
    S_Cards.ID_Student = Students.ID 
AND S_Cards.ID_Book = Books.ID 
AND Books.ID_Author = Authors.ID
GROUP BY Students.FirstName & " " & Students.LastName, 
    Books.Name AS [Book]

我建议您开始使用显式而不是隐式连接。在大多数情况下，MS Access 有更好的选择。

<...>
FROM Students
INNER JOIN S_Cards
ON Students.ID = S_Cards.ID_Student

或者

<...>
FROM Students
LEFT JOIN S_Cards
ON Students.ID = S_Cards.ID_Student

查询设计窗口将允许您使用正确的语法构建您的 JOIN。只需将连接字段从一个表拖放到下一个表，然后选择您需要的连接类型。