表名: Transactions
+-------------+------+
| Column Name | Type |
+-------------+------+
| account_id | int |
| day | date |
| type | ENUM |
| amount | int |
+-------------+------+
(account_id, day) 是该Transactions表的主键.
表中的每行数据表示一次交易的信息, 包括此次交易的账号(account_id), 交易类型(type), 交易发生时间(day), 交易发生金额(amount).
其中交易类型(type)字段包括了两种行为：存入 ('Deposit'), 取出('Withdraw').
请写出能够返回用户每次交易完成后的账户余额. 我们约定所有用户在进行交易前的账户余额都为0， 并且保证所有交易行为后的余额不为负数。
返回的结果请依次按照 账户（account_id), 日期( day ) 进行升序排序 .
查询结果的格式请参照以下测试样例.
测试样例1:
输入: Transactions 表: +------------+------------+----------+--------+ | account_id | day | type | amount | +------------+------------+----------+--------+ | 1 | 2021-11-07 | Deposit | 2000 | | 1 | 2021-11-09 | Withdraw | 1000 | | 1 | 2021-11-11 | Deposit | 3000 | | 2 | 2021-12-07 | Deposit | 7000 | | 2 | 2021-12-12 | Withdraw | 7000 | +------------+------------+----------+--------+ 输出: +------------+------------+---------+ | account_id | day | balance | +------------+------------+---------+ | 1 | 2021-11-07 | 2000 | | 1 | 2021-11-09 | 1000 | | 1 | 2021-11-11 | 4000 | | 2 | 2021-12-07 | 7000 | | 2 | 2021-12-12 | 0 | +------------+------------+---------+ 解释: 账户1: - 初始金额为 0. - 2021-11-07 --> 存入2000. 余额变为 0 + 2000 = 2000. - 2021-11-09 --> 取出1000. 余额变为 2000 - 1000 = 1000. - 2021-11-11 --> 存入3000. 余额变为 1000 + 3000 = 4000. 账户2: - 初始金额为 0. - 2021-12-07 --> 存入7000. 余额变为 0 + 7000 = 7000. - 2021-12-12 --> 取出 7000. 余额变为 7000 - 7000 = 0.