Get top n counted rows in table within group [MySQL]

I’m trying to get just top 3 selling products grouped within categories (just top 3 products by occurrence in transactions (id) count(id) by each category). I was searching a lot for possible solution but with no result. It looks like it is a bit tricky in MySQL since one can’t simply use top() function and so on. Sample data structure bellow:

+--------+------------+-----------+
|     id |category_id | product_id|
+--------+------------+-----------+
| 1      | 10         | 32        |
| 2      | 10         | 34        |
| 3      | 10         | 32        |
| 4      | 10         | 21        |
| 5      | 10         | 100       |
| 6      | 7          | 101       |
| 7      | 7          | 39        |
| 8      | 7          | 41        |
| 9      | 7          | 39        |
+--------+------------+-----------+

Answer

In earlier versions of MySQL, I would recommend using variables:

select cp.*
from (select cp.*,
             (@rn := if(@c = category_id, @rn + 1,
                        if(@c := category_id, 1, 1)
                       )
             ) as rn
      from (select category_id, product_id, count(*) as cnt
            from mytable
            group by category_id, product_id
            order by category_id, count(*) desc
           ) cp cross join
           (select @c := -1, @rn := 0) params
     ) cp
where rn <= 3;