subtract with previous record

DROP TABLE IF EXISTS stok;
CREATE TABLE IF NOT EXISTS stok (
  id smallint(4) unsigned NOT NULL AUTO_INCREMENT,
  item varchar(50) NOT NULL,
  periode DATE,
  jumlah int(11) unsigned NOT NULL,
  PRIMARY KEY (id)
) ENGINE=MyISAM;

INSERT INTO stok(item,periode,jumlah)
VALUES
('garam','2016-12-31',100),
('garam','2017-12-31',200),
('kecap','2015-10-15',200),
('kecap','2016-10-15',450),
('sambel','2016-09-07',300),
('sambel','2017-09-07',300),
('sambel','2018-09-07',500);

SELECT * FROM stok;

+----+--------+------------+--------+
| id | item   | periode    | jumlah |
+----+--------+------------+--------+
|  1 | garam  | 2016-12-31 |    100 |
|  2 | garam  | 2017-12-31 |    200 |
|  3 | kecap  | 2015-10-15 |    200 |
|  4 | kecap  | 2016-10-15 |    450 |
|  5 | sambel | 2016-09-07 |    300 |
|  6 | sambel | 2017-09-07 |    300 |
|  7 | sambel | 2018-09-07 |    500 |
+----+--------+------------+--------+

SELECT
a.periode,
IF(@prev_item=a.item, a.jumlah - @prev_jumlah,0) AS increseament,
@prev_jumlah:=a.jumlah AS jumlah,
@prev_item:=a.item AS item
FROM
stok a, (SELECT @prev_item:=0, @prev_jumlah:=0) b
ORDER BY a.item,a.periode;

+------------+--------------+--------+--------+
| periode    | increseament | jumlah | item   |
+------------+--------------+--------+--------+
| 2016-12-31 |            0 |    100 | garam  |
| 2017-12-31 |          100 |    200 | garam  |
| 2015-10-15 |            0 |    200 | kecap  |
| 2016-10-15 |          250 |    450 | kecap  |
| 2016-09-07 |            0 |    300 | sambel |
| 2017-09-07 |            0 |    300 | sambel |
| 2018-09-07 |          200 |    500 | sambel |
+------------+--------------+--------+--------+

SELECT
a.periode,
CASE WHEN @prev_item=a.item THEN  a.jumlah - @prev_jumlah ELSE 0 END AS increseament,
@prev_jumlah:=a.jumlah AS jumlah,
@prev_item:=a.item AS item
FROM
stok a, (SELECT @prev_item:=0, @prev_jumlah:=0) b
ORDER BY a.item,a.periode;