Compare all records of a n-n relationship - mysql

Question

I have three tables, A,B and C
Tables A and B have an ID and other fields, table C is a n-n relationship of A and B (C contains only IDS of A and B with primary key(id_a, id_b) and foreign key).
Now I need to verify if a row of A has the same associations of another row of A.
EXAMPLE
A = [id_a] = [1,2,3,4]
B = [id_b] = [1,2,3,4]
C = [id_a, id_b] = [[1,1],[1,3],[2,1],[2,3],[3,3]]
In this case, I need to extract only records where id_a are 1 and 2, because they are both associated with the same row of B (id_b 1 and 3).
id_a=3 is not the same as id_a=1/2, because it isn’t associated with id_b=1
This is for a new Ubuntu 18.04 server, running MySQL, PHP 7 (LAMP stack)
Example with id_a=1
SELECT id_a
FROM C
WHERE id_b IN (SELECT id_b FROM B WHERE id_a=1)
GROUP BY id_a;

Answer 1

The simplest method for an exact match uses group_concat():
select c.id_a
from (select c.id_a, group_concat(c.id_b order by c.id_b) as id_bs
from c
group by c.id_a
) c join
(select group_concat(c.id_b order by c.id_b) as id_bs
from c
where c.id_a = #id_a
) ac
on c.id_bs = ac.id_bs