running stock out

DROP TABLE IF EXISTS tabel;
CREATE TABLE IF NOT EXISTS tabel(
    id INT AUTO_INCREMENT PRIMARY KEY,
    tgl DATE,
    stok INT
);

INSERT INTO tabel(tgl,stok)
VALUES
('2012-08-01',50),
('2012-08-02',45),
('2012-08-03',40),
('2012-08-04',30);

SELECT * FROM tabel;
+----+------------+------+
| id | tgl        | stok |
+----+------------+------+
|  1 | 2012-08-01 |   50 |
|  2 | 2012-08-02 |   45 |
|  3 | 2012-08-03 |   40 |
|  4 | 2012-08-04 |   30 |
+----+------------+------+

SELECT d.id,d.tgl,d.stok,IFNULL(e.stok,d.stok)-d.stok AS keluar
FROM
(
SELECT a.id AS id1, MAX(b.id) AS id2
FROM
tabel a
LEFT JOIN tabel b ON b.id<a.id
GROUP BY a.id
) c
LEFT JOIN tabel d ON d.id=c.id1
LEFT JOIN tabel e ON e.id=c.id2;

+------+------------+------+--------+
| id   | tgl        | stok | keluar |
+------+------------+------+--------+
|    1 | 2012-08-01 |   50 |      0 |
|    2 | 2012-08-02 |   45 |      5 |
|    3 | 2012-08-03 |   40 |      5 |
|    4 | 2012-08-04 |   30 |     10 |
+------+------------+------+--------+

//--
$sql="SELECT * FROM tabel";
$result=$db->query($sql);
$data=array();
$i=-1;
foreach($result as $row){
    $data[++$i]=$row;
    $data[$i]['keluar']=($i==0?0:$data[$i-1]['stok']);
    echo "{$data[$i]['id']} | {$data[$i]['tgl']} | {$data[$i]['stok']} | {$data[$i]['keluar']}<br>";
}
//--