我需要为每行（每天 1 行）获取 7 天的滚动总和。
例如：
| Date | Count | 7-Day Rolling Sum |
------------------------------------------
| 2016-02-01 | 1 | 1
| 2016-02-02 | 1 | 2
| 2016-02-03 | 2 | 4
| 2016-02-04 | 2 | 6
| 2016-02-05 | 2 | 8
| 2016-02-06 | 2 | 10
| 2016-02-07 | 2 | 12
| 2016-02-08 | 2 | 13 --> here we start summing from 02-02
| 2016-02-09 | 2 | 14 --> here we …