SELECT products_2.id_good,products_2.product_name
    ,buying.quantity_buying,buying.summ_good_buying
    ,selling.quantity_order,selling.summ_good_order
FROM
(
  SELECT id_good_buying,product_name_buying FROM buying
  UNION
  SELECT id_good_selling,product_name_selling FROM selling
  ) products_2
LEFT JOIN
(
  SELECT id_good_buying,product_name_buying,SUM('quantity_buying') ,SUM('summ_good_buying') 
  FROM buying
  GROUP BY id_good_buying,product_name_buying
) buying
LEFT JOIN
(
  SELECT id_good_selling,product_name_selling,SUM('quantity_order') ,SUM('summ_good_order')
  FROM selling
  GROUP BY id_good_selling,product_name_selling
) selling
on products_2.id_good=buying.id_good_buying
and products_2.product_name=buying.product_name_buying
on products_2.id_good=selling.id_good_selling
and products_2.product_name=selling.product_name_selling

SELECT t1.id_good,t1.product_name
    ,t2.quantity_buying,t2.summ_good_buying
    ,t3.quantity_order,t3.summ_good_order
FROM
(
  SELECT id_good,product_name FROM buying
  UNION
  SELECT id_good,product_name FROM selling
  ) t1
LEFT JOIN
(
  SELECT id_good,product_name,sum(amount) quantity_buying,sum(summ) summ_good_buying 
  FROM buying
  GROUP BY id_good,product_name
) t2
LEFT JOIN
(
  SELECT id_good,product_name,sum(amount) quantity_order,sum(summ) summ_good_order
  FROM selling
  GROUP BY id_good,product_name
) t3
on t1.id_good=t2.id_good
and t1.product_name=t2.product_name
on t1.id_good=t3.id_good
and t1.product_name=t3.product_name