MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

select version();


version()


8.0.30


CREATE TABLE anillas(
  id_anilla int AUTO_INCREMENT PRIMARY KEY,
  anilla varchar(7),
  codigo int
  );
INSERT INTO anillas (anilla, codigo) VALUES
  ('A1',123),('A1',123),('A1',122),
  ('A2',321),('A2',321),
  ('A3',654),('A3',123);
SELECT * FROM anillas;


id_anilla
anilla
codigo


1
A1
123

2
A1
123

3
A1
122

4
A2
321

5
A2
321

6
A3
654

7
A3
123


SELECT anilla,GROUP_CONCAT(DISTINCT codigo)
  FROM anillas
  GROUP BY 1
  HAVING COUNT(DISTINCT codigo)>1;


anilla
GROUP_CONCAT(DISTINCT codigo)


A1
122,123

A3
123,654

`id_anilla`	`anilla`	`codigo`
`1`	`A1`	`123`
`2`	`A1`	`123`
`3`	`A1`	`122`
`4`	`A2`	`321`
`5`	`A2`	`321`
`6`	`A3`	`654`
`7`	`A3`	`123`

`anilla`	`GROUP_CONCAT(DISTINCT codigo)`
`A1`	`122,123`
`A3`	`123,654`