mysql> select * from `凭证表`;
+-----------+--------+--------+--------+--------+
| 日期      | 凭证字 | 凭证号 | 借金额 | 贷金额 |
+-----------+--------+--------+--------+--------+
| 2006-9-13 | 转     | 1      | ？     | ？     |
| 2006-9-14 | 付     | 1      | ？     | ？     |
+-----------+--------+--------+--------+--------+
2 rows in set (0.00 sec)

mysql> select * from `凭证明细表`;
+------------+--------+--------+------+----------+----------+
| 日期
日期 | 凭证字 | 凭证号 | 序号 | 借贷方向 | 发生金额 |
+------------+--------+--------+------+----------+----------+
| 2006-9-13  | 转     | 1      | 1    | 借       | 100      |
| 2006-9-13  | 转     | 1      | 2    | 借       | 100      |
| 2006-9-13  | 转     | 1      | 3    | 贷       | 200      |
| 2006-9-14  | 付     | 1      | 1    | 借       | 300      |
| 2006-9-14  | 付     | 1      | 2    | 贷       | 300      |
+------------+--------+--------+------+----------+----------+
5 rows in set (0.00 sec)

mysql> update `凭证表` as a
    -> set `借金额` = (select sum(`发生金额`) from `凭证明细表` as b where a.`日
期` = b.`日期` and `借贷方向` = '借'),
    ->     `贷金额` = (select sum(`发生金额`) from `凭证明细表` as b where a.`日
期` = b.`日期` and `借贷方向` = '贷');
Query OK, 2 rows affected (0.03 sec)
Rows matched: 2  Changed: 2  Warnings: 0

mysql> select * from `凭证表`;
+-----------+--------+--------+--------+--------+          ;
| 日期      | 凭证字 | 凭证号 | 借金额 | 贷金额 |
+-----------+--------+--------+--------+--------+
| 2006-9-13 | 转     | 1      | 200    | 200    |
| 2006-9-14 | 付     | 1      | 300    | 300    |
+-----------+--------+--------+--------+--------+
2 rows in set (0.00 sec)