Group activities in activity feed by users and products

On my website I'm building an activity feed where I got the basics done and are now making some improvements here and there.

One of them is the grouping. I've built it so that if multiple users buys the same 2 or more products the activity feed shows it as one activity: "user1 & user2 bought product 1, 2 & 3".

The problem is that if user1 buys product 1 & 2 and user2 buys product 1, 2 & 3 it appears as two different activities: "user1 bought product 1 & 2" AND "user2 bought product 1, 2 & 3".

I would like it to show as "user1 & user2 bought product 1 & 2" and then another saying "user2 bought product 3"

I've made a simplified sqlfiddle of what I've got so far. Try changing it by deleting NULL, 'Bought', 1, 3

Is this something that is possible to do in MySQL or should I do it in PHP?

You need to group users by activity and product in the first step, and then group products by the groups of users.

I use ORDER BY with GROUP_CONCAT(), so that the aggregated usernames are concatenated in the same order.

SELECT x.usernames
      ,x.activity
      ,GROUP_CONCAT(p.product_name ORDER BY p.product_name) AS product_names
FROM   products p
JOIN  (
   SELECT a.activity
         ,a.productId
         ,GROUP_CONCAT(u.name ORDER BY u.name) AS usernames
   FROM   activities a
   LEFT   JOIN users u ON u.id = a.userId
   GROUP  BY 1, 2
   ) x ON x.productId = p.id
GROUP  BY 1, 2
ORDER  BY 1, 2;

-> sqlfiddle

Returns:

USERNAMES  | ACTIVITY   | PRODUCT_NAMES
-----------+------------+------------------------------
John       | Bought     | The Dark Knight Rises
John,Peter | Bought     | Batman Begins,The Dark Knight

StackExchange Database Administrators Q#31515, answer score: 2