SELECT COUNT(*) as `count`, s.fk_product as `ID` from stocks as s WHERE s.sold = 0 group by s.fk_product

SELECT products.id as 'ID', t.my_count
  FROM products
  INNER JOIN (
      SELECT fk_product as id, COUNT(*)  my_count
      from stocks
     WHERE stocks.sold = 0
     group by fk_product

  ) t on t.id = products.id