Извадете количеството от предишния ред MySQL

можете да използвате това и без JOIN към таблицата:изходящият SELECT е само за скриване на някои колони.

SELECT Quantity, `timestamp`, sold_qty
FROM (
    SELECT i.*,
    @sold_qty := GREATEST(@last_qty - i.`Quantity`,0) as sold_qty,
    @last_qty := i.`Quantity` as last_qty
    FROM InventoryTest_history i
    CROSS JOIN ( SELECT @last_qty := '', @sold_qty := 0) as init
    ORDER BY `timestamp`
    ) as result;

мостра

mysql> SELECT * from InventoryTest_history;

+----+-------------+----------+---------------------+
| id | SKU         | Quantity | timestamp           |
+----+-------------+----------+---------------------+
|  1 | (L) U-Joint |      985 | 2016-12-27 10:08:58 |
|  2 | (L) U-Joint |      960 | 2016-12-28 10:09:52 |
|  3 | (L) U-Joint |      955 | 2016-12-29 16:01:02 |
+----+-------------+----------+---------------------+
3 rows in set (0,02 sec)

mysql> SELECT Quantity, `timestamp`, sold_qty
    -> FROM (
    ->     SELECT i.*,
    ->     @sold_qty := GREATEST(@last_qty - i.`Quantity`,0) as sold_qty,
    ->     @last_qty := i.`Quantity` as last_qty
    ->     FROM InventoryTest_history i
    ->     CROSS JOIN ( SELECT @last_qty := '', @sold_qty := 0) as init
    ->     ORDER BY `timestamp`
    ->     ) as result;
+----------+---------------------+----------+
| Quantity | timestamp           | sold_qty |
+----------+---------------------+----------+
|      985 | 2016-12-27 10:08:58 |        0 |
|      960 | 2016-12-28 10:09:52 |       25 |
|      955 | 2016-12-29 16:01:02 |        5 |
+----------+---------------------+----------+
3 rows in set (0,00 sec)

mysql> SELECT i.*,
    -> @sold_qty := GREATEST(@last_qty - i.`Quantity`,0) as sold_qty,
    -> @last_qty := i.`Quantity` as last_qty
    -> FROM InventoryTest_history i
    -> CROSS JOIN ( SELECT @last_qty := '', @sold_qty := 0) as init
    -> ORDER BY `timestamp`;
+----+-------------+----------+---------------------+----------+----------+
| id | SKU         | Quantity | timestamp           | sold_qty | last_qty |
+----+-------------+----------+---------------------+----------+----------+
|  1 | (L) U-Joint |      985 | 2016-12-27 10:08:58 |        0 |      985 |
|  2 | (L) U-Joint |      960 | 2016-12-28 10:09:52 |       25 |      960 |
|  3 | (L) U-Joint |      955 | 2016-12-29 16:01:02 |        5 |      955 |
+----+-------------+----------+---------------------+----------+----------+
3 rows in set (0,00 sec)

mysql>