I need to calculate the BLACKLOG_M value corresponding to the Snapdate of the first of the month. It becomes a constant value repeated for each line of the month
select * from test
| SNAP | PERI | ANNEE | MOIS | BCKlog | BCKlog-1 |
| 01/09/2018 | 201808 | 2018 | 8 | NULL | NULL |
| 02/09/2018 | 201809 | 2018 | 9 | NULL | NULL |
| 03/09/2018 | 201809 | 2018 | 9 | NULL | NULL |
| 04/09/2018 | 201809 | 2018 | 9 | NULL | NULL |
| 05/09/2018 | 201809 | 2018 | 9 | NULL | NULL |
| 06/09/2018 | 201809 | 2018 | 9 | 25279.3465475233 | NULL |
| 07/09/2018 | 201809 | 2018 | 9 | NULL | NULL |
| 08/09/2018 | 201809 | 2018 | 9 | NULL | NULL |
| 09/09/2018 | 201809 | 2018 | 9 | NULL | NULL |