01.SELECT products_id, SUM(qtyOld + qtyCurr) total
02.FROM (
03.SELECT products_id, 0 qtyOld, (order_p - totalstock_products) qtyCurr
04.FROM table
05.WHERE DATE_FORMAT(date_insert, '%Y-%m') = DATE_FORMAT(NOW(), '%Y-%m')
06.
07.UNION
08.
09.SELECT products_id, totalstock_products qtyOld, 0 qtyCurr
10.FROM table
11.WHERE DATE_FORMAT(date_insert, '%Y-%m') = DATE_FORMAT(SUBDATE(NOW(),INTERVAL 1 MONTH), '%Y-%m')
12.) U
13.GROUP BY products_id
