Find duplicated values on array column

Question

I have a table with a array column like this:

my_table
id   array
--   -----------
1    {1, 3, 4, 5}
2    {19,2, 4, 9}
3    {23,46, 87, 6}
4    {199,24, 93, 6}

And i want as result what and where is the repeated values, like this:

value_repeated    is_repeated_on
--------------    -----------
4                 {1,2}
6                 {3,4}

Is it possible? I don't know how to do this. I don't how to start it! I'm lost!

Answer 1

Use unnest to convert the array to rows, and then array_agg to build an array from the ids

It should look something like this:

SELECT v AS value_repeated,array_agg(id) AS is_repeated_on FROM 
(select id,unnest(array) as v from my_table) 
GROUP by v HAVING Count(Distinct id) > 1

Note that HAVING Count(Distinct id) > 1 is filtering values that don't appear even once

Answer 2

The clean way to call a set-returning function like unnest() is in a LATERAL join, available since Postgres 9.3:

SELECT value_repeated, array_agg(id) AS is_repeated_on
FROM   my_table
     , unnest(array_col) value_repeated
GROUP  BY value_repeated
HAVING count(*) > 1
ORDER  BY value_repeated;  -- optional

About LATERAL:

• Call a set-returning function with an array argument multiple times

There is nothing in your question to rule out shortcut duplicates (the same element more than once in the same array (like I@MSoP commented), so it must be count(*), not count (DISTINCT id).