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| month | customer_id |
|---|---|
| 2021-01-01 | A |
| 2021-02-01 | A |
| 2021-03-01 | A |
| 2021-02-01 | B |
| 2021-03-01 | B |
| 2021-04-01 | B |
| 2021-06-01 | C |
| 2021-07-01 | C |
| 2021-08-01 | C |
| 2021-03-01 | D |
| 2021-04-01 | D |
| 2021-05-01 | D |
SELECT 12