Mysql借记贷方余额计算

Question

我有以下表格用于我用php mysql制作的小型金融网络应用程序。 我需要帮助计算mysql中的借方和贷方余额。

1.表：条目

+----+---------+------------+------------+
| ID | TYPE_ID | NARRATION  |    DATE    |
+----+---------+------------+------------+
|  1 |       3 | Test Input | 2015-01-05 |
|  2 |       2 | Test Input | 2015-02-07 |
|  3 |       2 | Test Input | 2015-04-02 |
+----+---------+------------+------------+

2.Table：Entry_items

+----+----------+-----------+-------+--------+
| ID | ENTRY_ID | LEDGER_ID | TYPE  | AMOUNT |
+----+----------+-----------+-------+--------+
|  1 |        1 |         1 | D     |   2000 |
|  3 |        1 |         2 | C     |   2000 |
|  4 |        2 |         2 | D     |    550 |
|  5 |        2 |         1 | C     |    550 |
+----+----------+-----------+-------+--------+

所以我能够用分类帐ID = 1

列出金额

SELECT e.date AS date,
 i.type AS type,
 i.amount AS amount
FROM entries AS e LEFT JOIN entry_items AS i ON e.id=i.entry_id
WHERE i.ledger_id = '1'

结果如下：

+------------+---+------+
| 2015-01-05 | D | 2000 |
| 2015-02-07 | C |  550 |
+------------+---+------+

到目前为止，我找不到计算余额的方法。

我想要的结果是：

+------------+------+---------+---------+
|    Date    | Type | Amount  | Balance |
+------------+------+---------+---------+
| 2015-01-05 | D    |    2000 |    2000 |
| 2015-02-07 | C    |     550 |    1450 |
+------------+------+---------+---------+

我完全迷失了当前的代码：

SELECT e.date AS date,
 i.type AS type,
 i.amount AS amount,
 SUM(COALESCE(CASE WHEN type = 'D' THEN amount END,0)) 
 - SUM(COALESCE(CASE WHEN type = 'C' THEN amount END,0)) balance
FROM entries AS e LEFT JOIN entry_items AS i ON e.id=i.entry_id
WHERE i.ledger_id = '1'