Да уж ))
Ну вообще думаю будет как-то так :
Пример на MSSQL
Пример на Psql13
Результат:
+========+============+=====================+
| userid | regdate | newdate |
+========+============+=====================+
| 1 | 2020-11-01 | 2020-11-01 00:00:00 |
+--------+------------+---------------------+
| 1 | 2020-11-01 | 2020-12-01 00:00:00 |
+--------+------------+---------------------+
| 1 | 2020-11-01 | 2020-12-31 00:00:00 |
+--------+------------+---------------------+
| 1 | 2020-11-01 | 2021-01-30 00:00:00 |
+--------+------------+---------------------+
| 1 | 2020-11-01 | 2021-03-01 00:00:00 |
+--------+------------+---------------------+
| 2 | 2020-12-01 | 2020-12-01 00:00:00 |
+--------+------------+---------------------+
| 2 | 2020-12-01 | 2020-12-31 00:00:00 |
+--------+------------+---------------------+
| 2 | 2020-12-01 | 2021-01-30 00:00:00 |
+--------+------------+---------------------+
| 2 | 2020-12-01 | 2021-03-01 00:00:00 |
+--------+------------+---------------------+
