MariaDB 10.2

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create table products (
    id int auto_increment primary key,
    product_category_id int
)
insert into products (product_category_id)
    values (1), (2), (1), (3), (1), (4), (4), (4);
select *
from products


id
product_category_id


1
1

2
2

3
1

4
3

5
1

6
4

7
4

8
4


select p.*
from (select p.*,
             (@rn := if(@c = product_category_id, @rn + 1,
                        if(@c := product_category_id, 1, 1)
                       )
             ) as rn
      from products p cross join
           (select @c := -1, @rn := 0) params
      order by p.product_category_id
     ) p
where rn <= 2


id
product_category_id
rn


1
1
1

3
1
2

2
2
1

4
3
1

6
4
1

7
4
2