Find rows with duplicate values ​​in a column

Question

I have a table author_data:

 author_id | author_name
 ----------+----------------
 9         | ernest jordan
 14        | k moribe
 15        | ernest jordan
 25        | william h nailon 
 79        | howard jason
 36        | k moribe

Now I need a result like:

 author_id | author_name                                                  
 ----------+----------------
 9         | ernest jordan
 15        | ernest jordan     
 14        | k moribe 
 36        | k moribe

That is, I need author_idfor names that have repeated occurrences. I tried this statement:

select author_id,count(author_name)
from author_data
group by author_name
having count(author_name)>1

But it does not work. How can i get this?

Answer 1

I suggest a window function in a subquery:

SELECT author_id, author_name  -- omit the name here, if you just need ids
FROM (
   SELECT author_id, author_name
        , count(*) OVER (PARTITION BY author_name) AS ct
   FROM   author_data
   ) sub
WHERE  ct > 1;

You will learn the basic aggregate function count(). It can be turned into a window function by adding a sentence OVER- just like any other aggregated function.

, . .

(v.8.3 ) - - :

SELECT author_id, author_name  -- omit name, if you just need ids
FROM   author_data a
WHERE  EXISTS (
   SELECT 1
   FROM   author_data a2
   WHERE  a2.author_name = a.author_name
   AND    a2.author_id <> a.author_id
   );

, author_name.

Answer 2

. Author_IDs .

.

SELECT author_id, author_name
FROM author_data
WHERE author_id in (select author_id
        from author_data
        group by author_name
        having count(author_name)>1)

Answer 3

You can join the table by yourself, which can be achieved using one of the following queries:

SELECT a1.author_id, a1.author_name
FROM authors a1
CROSS JOIN authors a2
  ON a1.author_id <> a2.author_id
  AND a1.author_name = a2.author_name;

-- 9 |ernest jordan
-- 15|ernest jordan
-- 14|k moribe
-- 36|k moribe

--OR

SELECT a1.author_id, a1.author_name
FROM authors a1
INNER JOIN authors a2
  WHERE a1.author_id <> a2.author_id
  AND a1.author_name = a2.author_name;

-- 9 |ernest jordan
-- 15|ernest jordan
-- 14|k moribe
-- 36|k moribe