我有一个带有列date和的表格time_spent。我想为每个日期D查找一段时间内“
time_spent”值的总和：（D-7-D），即。过去一周+当前日期。

我找不到解决办法，因为我只能找到一个总和的示例，而不是一个可变时间段内的总和。

这是一个数据集示例：

CREATE TABLE rolling_total
(
  date date,
  time_spent int
);

INSERT INTO rolling_total VALUES ('2013-09-01','2'),
('2013-09-02','1'),
('2013-09-03','3'),
('2013-09-04','4'),
('2013-09-05','2'),
('2013-09-06','5'),
('2013-09-07','3'),
('2013-09-08','2'),
('2013-09-09','1'),
('2013-09-10','1'),
('2013-09-11','1'),
('2013-09-12','3'),
('2013-09-13','2'),
('2013-09-14','4'),
('2013-09-15','6'),
('2013-09-16','1'),
('2013-09-17','2'),
('2013-09-18','3'),
('2013-09-19','4'),
('2013-09-20','1'),
('2013-09-21','6'),
('2013-09-22','5'),
('2013-09-23','3'),
('2013-09-24','1'),
('2013-09-25','5'),
('2013-09-26','2'),
('2013-09-27','1'),
('2013-09-28','4'),
('2013-09-29','3'),
('2013-09-30','2')

结果看起来像：

date       | time_spent   | rolling_week_total
2013-09-01 |           2  |         2
2013-09-02 |           1  |         3
2013-09-03 |           3  |         6
2013-09-04 |           4  |        10
2013-09-05 |           2  |        12
2013-09-06 |           5  |        17
2013-09-07 |           3  |        20
2013-09-08 |           2  |        22
// now we omit values that are older than seven days
2013-09-09 |           1  |        21
2013-09-10 |           1  |        21
...