You haven't specified your desired output. But if you're wanting a comma separated list by group you can do something like this answer from the below StackOverflow threads:
MySQL
SELECT t.group_id,
GROUP_CONCAT(t.product_id) AS product_id_group
FROM [YOURTABLE] t
GROUP BY t.id, t.product_id;
MySQL Results as comma separated list
SQL Sever
SELECT group_id, product_id =
STUFF((SELECT ', ' + product_id
FROM [YOURTABLE] t1
WHERE t1.group_id = t2.group_id
FOR XML PATH('')), 1, 2, '')
FROM [YOURTABLE] t2
GROUP BY group_id
SQL Server : GROUP BY clause to get comma-separated values
This will return a result set like:
group_id product_id
2 65, 50, 30, 60, 42
5 40, 42
6 30, 65, 60
7 90, 40