MySQL 8.0

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CREATE TABLE test (product1 INT, product2 INT);
INSERT INTO test (product1, product2)
VALUES
(1,2),
(2,3),
(3,4),
(3,5),
(4,6),
(7,8),
(8,9),
(10,11);
WITH RECURSIVE 
products AS (
SELECT product1 p1, product2 p2
FROM test
UNION
SELECT product2 p1, product1 p2
FROM test
UNION
SELECT p.p1, t.product2
FROM products p, test t
WHERE p.p2 = t.product1
UNION
SELECT p.p1, t.product1
FROM products p, test t
WHERE p.p2 = t.product2
)
SELECT p1 product, GROUP_CONCAT(p2 ORDER BY p2) alternatives
FROM products 
GROUP BY p1
ORDER BY p1;


product
alternatives


1
1,2,3,4,5,6

2
1,2,3,4,5,6

3
1,2,3,4,5,6

4
1,2,3,4,5,6

5
1,2,3,4,5,6

6
1,2,3,4,5,6

7
7,8,9

8
7,8,9

9
7,8,9

10
10,11

11
10,11

product	alternatives
1	1,2,3,4,5,6
2	1,2,3,4,5,6
3	1,2,3,4,5,6
4	1,2,3,4,5,6
5	1,2,3,4,5,6
6	1,2,3,4,5,6
7	7,8,9
8	7,8,9
9	7,8,9
10	10,11
11	10,11