select q.*
from (select q.*,
             (@rn := if(@c = category_id, @rn + 1,
                        if(@c := category_id, 1, 1)
                       )
             ) as rn
      from questions q cross join
           (select @rn := 0, @c := -1) params
      order by category_id rand()
     ) q
where rn <= 5;