Group activities in activity feed by users and products

Question

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?