本章节重点在于讲解求和、计数的窗口函数应用。
题目背景如下：
有一个订单信息表(order_info)，简况如下:
| id | user_id | product_name | status | client_id | date |
| 1 | 557336 | C++ | no_completed | 1 | 2025-10-10 |
| 2 | 230173543 | Python | completed | 2 | 2025-10-12 |
| 3 | 57 | JS | completed | 3 | 2025-10-23 |
| 4 | 57 | C++ | completed | 3 | 2025-10-23 |
| 5 | 557336 | Java | completed | 1 | 2025-10-23 |
| 6 | 57 | Java | completed | 1 | 2025-10-24 |
| 7 | 557336 | C++ | completed | 1 | 2025-10-25 |
| 8 | 557336 | Python | completed | 1 | 2025-10-26 |