How do I find duplicate entries in a database table?

Question

The following query will display all Dewey Decimal numbers that have been duplicated in the "book" table:

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

However, what I'd like to do is have my query display the name of the authors associated with the duplicated entry (the "book" table and "author" table are connected by "author_id"). In other words, the query above would yield the following:

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

What I'd like the results to display is something similar to the following:

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

Any help you can provide is greatly appreciated. And, in case it comes into play, I'm using a Postgresql DB.

UPDATE: Please note that "author_last_name" is not in the "book" table.

Answer 1

A nested query can do the job.

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 

(I don't know if this is the fastest way to achieve what you want.)

Update: Here is my data

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 

And here is the result of the above query:

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