Question

问题！我正在尝试列出所有书籍及其相应的图书主题，平均成本和常规费用。

到目前为止，我的查询是......

SELECT BOOK_SUBJECT, AVG( BOOK_COST ) 
FROM BOOK 
GROUP BY BOOK_SUBJECT

此查询为我提供了四组总费用的平均值。 Final out put should look like this我需要引入BOOK_NUM，BOOK_TITLE，BOOK_SUBJECT，BOOK_COST，但我无法弄明白。有人可以帮忙吗？相关子查询？

Answer 1

使用子查询执行此操作：

SELECT BOOK.BOOK_NUM, BOOK.BOOK_TITLE, BOOK.BOOK_SUBJECT, BOOK.BOOK_COST, T.AVG_COST
FROM BOOK
INNER JOIN (
    SELECT BOOK_SUBJECT, AVG(BOOK_COST) AVG_COST
    FROM BOOK
    GROUP BY BOOK_SUBJECT
) T ON BOOK.BOOK_SUBJECT = T.BOOK_SUBJECT

Answer 2

请尝试以下方法......

SELECT BOOK_NUM AS Number,
       BOOK_TITLE AS Title,
       BOOK.BOOK_SUBJECT AS Subject,
       BOOK_COST AS Cost,
       AVG_BOOK_COST AS 'Avg Cost'
FROM BOOK
JOIN ( SELECT BOOK_SUBJECT AS BOOK_SUBJECT,
              AVG( BOOK_COST ) AS AVG_BOOK_COST
       FROM BOOK
       GROUP BY BOOK_SUBJECT
     ) AS SUBJECT_AVG_FINDER ON BOOK.BOOK_SUBJECT = SUBJECT_AVG_FINDER.BOOK_SUBJECT
ORDER BY BOOK_NUM;

要计算每个主题的平均值，我们必须按BOOK对BOOK_SUBJECT的内容进行分组，然后使用AVG( BOOK_COST )查找每个组的平均值。但我们也希望避免在BOOK中对其他字段进行分组，而是显示Book中每条记录的指定字段，并在最后添加BOOK_SUBJECT的平均成本。这表明在INNER JOIN和子查询之间BOOK，用于查找每个主题的平均费用。

我使用以下代码查找BOOK ...

中列出的每个主题的平均平均费用

SELECT BOOK_SUBJECT AS BOOK_SUBJECT,
       AVG( BOOK_COST ) AS AVG_BOOK_COST
FROM BOOK
GROUP BY BOOK_SUBJECT

我们需要部分选择BOOK_SUBJECT，因为GROUP BY子句需要它，部分原因是我们需要它将此子查询生成的表连接到BOOK的未分组列表。< / p>

为AVG( BOOK_COST )提供AVG_BOOK_COST别名，可以更轻松地引用此生成的字段。

如果在单词JOIN之前没有连接类型，则大多数SQL版本将假设为INNER JOIN，尽管所有允许使用INNER JOIN并且有些需要您这样做。默认情况下，我和其他许多人只使用JOIN。

一旦执行了连接，来自BOOK的每条记录都会从我们的子查询中获得它的相应记录的副本（我已经给出了SUBJECT_AVG_FINDER的别名），留下每条记录有两个名为BOOK_SUBJECT的字段。为了不混淆您的SQL版本，我们必须指定表/子查询以及发生此类重复的字段名称，因此在整个语句的第三行中为BOOK.BOOK_SUBJECT。

根据您所需的最终输出图像，每个字段都有一个别名。

我假设不需要复制行号字段。如果这不正确，请另行说明。

最后，我通过添加第ORDER BY BOOK_NUM行来按照您想要的输出对结果进行排序。

作为提示，虽然允许，但您应该避免使用喊叫（即全大写）您的字段名称，表名称和别名＆＃39; （除非你被要求这样做），但仍然大喊SQL的东西（如SELECT，FROM，AS等）。通过提供关于如何使用每个单词的直观线索，这可以使语句更容易阅读和调试。我建议用以下方式呈现我们的SQL语句......

SELECT book_num AS Number,
       book_title AS Title,
       book.book_subject AS Subject,
       book_cost AS Cost,
       avg_book_cost AS 'Avg Cost'
FROM book
JOIN ( SELECT book_subject AS book_subject,
              AVG( book_cost ) AS avg_book_cost
       FROM book
       GROUP BY book_subject
     ) AS subject_avg_finder ON book.book_subject = subject_avg_finder.book_subject
ORDER BY book_num;

如果您有任何问题或意见，请随时发表评论。

Answer 3

使用此代码

SELECT BOOK_NUM, BOOK_TITLE, BOOK_SUBJECT, BOOK_COST,  
AVG(BOOK_COST) OVER(PARTITION BY BOOK_SUBJECT) AS AVG_COST  
FROM BOOK

相关子查询以查找某些组的AVG

3 个答案: