Hi there,
I have data that is structured somewhat as follows:
| Employee number | Month |
| 111 | 2022 01 |
| 222 | 2022 01 |
| 333 | 2022 01 |
| 444 | 2022 01 |
| 555 | 2022 01 |
| 666 | 2022 01 |
| 777 | 2022 01 |
| 888 | 2022 01 |
| 999 | 2022 01 |
| 111 | 2022 02 |
| 233 | 2022 02 |
| 333 | 2022 02 |
| 444 | 2022 02 |
| 455 | 2022 02 |
| 555 | 2022 02 |
| 666 | 2022 02 |
| 777 | 2022 02 |
| 888 | 2022 02 |
| 111 | 2022 03 |
| 233 | 2022 03 |
| 333 | 2022 03 |
| 444 | 2022 03 |
| 455 | 2022 03 |
| 555 | 2022 03 |
| 777 | 2022 03 |
| 888 | 2022 03 |
| 899 | 2022 03 |
As you can see, the data is a continuous set of employee numbers for multiple periods of time (Jan to March 2022). In January, all employees are present but as the months go on, so new ones are added (eg 233, 455 and 899) and some are deleted (eg 222, 999 and 666).
I am looking for a solution that will determine what values are in a current month that are already there in the previous month. So for example, 111 is there in Jan, Feb and Mar, therefore it is a yes for Feb and Mar. Whereas 233 is only there in Feb and Mar, so it is a yes for Mar but no for Feb (as it wasn't there in January.
Example of output.
| Employee number | Month | There previous month? |
| 111 | 2022 01 | |
| 222 | 2022 01 | |
| 333 | 2022 01 | |
| 444 | 2022 01 | |
| 555 | 2022 01 | |
| 666 | 2022 01 | |
| 777 | 2022 01 | |
| 888 | 2022 01 | |
| 999 | 2022 01 | |
| 111 | 2022 02 | Y |
| 233 | 2022 02 | N |
| 333 | 2022 02 | Y |
| 444 | 2022 02 | Y |
| 455 | 2022 02 | N |
| 555 | 2022 02 | Y |
| 666 | 2022 02 | Y |
| 777 | 2022 02 | Y |
| 888 | 2022 02 | Y |
| 111 | 2022 03 | Y |
| 233 | 2022 03 | Y |
| 333 | 2022 03 | Y |
| 444 | 2022 03 | Y |
| 455 | 2022 03 | Y |
| 555 | 2022 03 | Y |
| 777 | 2022 03 | Y |
| 888 | 2022 03 | Y |
| 899 | 2022 03 | N |
