具体来说，我有一个事件表，用于记录用户加入或离开团队的时间。它看起来像下面这样：
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| user | event | team | timestamp |
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| A | joined | 1 | 2016-1-1 |
| B | joined | 1 | 2016-1-1 |
| C | left | 1 | 2016-1-1 |
| C | joined | 2 | 2016-1-1 |
| A | left | 1 | 2016-1-2 |
| A | joined | 2 | 2016-1-2 |
| B | left | 1 | 2016-1-3 |
| A | left | …