Postgresql 如何在数据库表中查找重复条目

发布于01月13日

以下查询将显示"book"表中重复的所有杜威十进制数:

SELECT dewey_number, 
 COUNT(dewey_number) AS NumOccurrences
FROM book
GROUP BY dewey_number
HAVING ( COUNT(dewey_number) > 1 )

然而，我想做的是让我的查询显示与复制条目相关联的作者的姓名("book"表和"author"表通过"author_id"连接).换句话说，上述查询将产生以下结果:

dewey_number | NumOccurrences
------------------------------
5000         | 2
9090         | 3

我希望显示的结果类似于以下内容:

author_last_name | dewey_number | NumOccurrences
-------------------------------------------------
Smith            | 5000         | 2
Jones            | 5000         | 2
Jackson          | 9090         | 3
Johnson          | 9090         | 3
Jeffers          | 9090         | 3

非常感谢您提供的任何帮助.为了防止它发挥作用，我正在使用Postgresql数据库.

UPDATE:请注意，"作者姓氏"不在"图书"表中.

推荐答案

嵌套查询可以完成这项工作.

SELECT author_last_name, dewey_number, NumOccurrences
FROM author INNER JOIN
     ( SELECT author_id, dewey_number,  COUNT(dewey_number) AS NumOccurrences
        FROM book
        GROUP BY author_id, dewey_number
        HAVING ( COUNT(dewey_number) > 1 ) ) AS duplicates
ON author.id = duplicates.author_id

(我不知道这是否是实现目标的最快方式.)

更新:这是我的数据

SELECT * FROM author;
 id | author_last_name 
----+------------------
  1 | Fowler
  2 | Knuth
  3 | Lang

SELECT * FROM book;
 id | author_id | dewey_number |         title          
----+-----------+--------------+------------------------
  1 |         1 |          600 | Refactoring
  2 |         1 |          600 | Refactoring
  3 |         1 |          600 | Analysis Patterns
  4 |         2 |          600 | TAOCP vol. 1
  5 |         2 |          600 | TAOCP vol. 1
  6 |         2 |          600 | TAOCP vol. 2
  7 |         3 |          500 | Algebra
  8 |         3 |          500 | Undergraduate Analysis
  9 |         1 |          600 | Refactoring
 10 |         2 |          500 | Concrete Mathematics
 11 |         2 |          500 | Concrete Mathematics
 12 |         2 |          500 | Concrete Mathematics

以上查询结果如下:

 author_last_name | dewey_number | numoccurrences 
------------------+--------------+----------------
 Fowler           |          600 |              4
 Knuth            |          600 |              3
 Knuth            |          500 |              3
 Lang             |          500 |              2