CREATE TABLE `item_zone` (
	`item`	INTEGER NOT NULL,
	`zone`	INTEGER NOT NULL,
	`action_date`	TEXT NOT NULL,
	`price`	INTEGER NOT NULL,
	`days_price_established`	INTEGER
);
INSERT INTO `item_zone` VALUES (50374500, 1, 1512248400, 5, 10);
INSERT INTO `item_zone` VALUES (50374500, 1, 1513112400, 9, 1);
INSERT INTO `item_zone` VALUES (50374500, 1, 1513198800, 8, null);

update item_zone
    set days_price_established=(
        select round(
            (iz1.action_date-(
                SELECT iz2.action_date FROM item_zone iz2 WHERE iz2.oid<iz1.oid AND iz2.item=iz1.item ORDER by iz2.oid DESC limit 1)
            )/86400) 
      from item_zone iz1 WHERE iz1.oid=item_zone.oid) 
WHERE days_price_established IS NULL;