MySQL 8.0

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select version();


version()


8.0.30


create table prod_ch_val (prod_id int, ch_id int, val int);

insert into prod_ch_val values
(1,1,5),
(1,1,6),
(1,2,1),
(2,2,1),
(3,1,5),
(3,1,6),
(3,2,1),
(4,1,5),
(4,1,6),
(4,2,1),
(4,2,2),
(5,2,1);

Records: 12  Duplicates: 0  Warnings: 0


WITH param AS (
  SELECT 5 AS prod_id -- ИД продукта которому ищем сопутствующие
)
SELECT p2.prod_id
  FROM param, prod_ch_val p1, prod_ch_val p2
  WHERE p1.prod_id = param.prod_id
    AND p1.ch_id=p2.ch_id
    AND p1.val = p2.val
    AND p1.prod_id != p2.prod_id
    AND (SELECT count(1) FROM prod_ch_val t WHERE t.prod_id=p2.prod_id) = (SELECT count(1) FROM prod_ch_val t WHERE t.prod_id=p1.prod_id)
GROUP BY p2.prod_id


prod_id


2