select x.monthly as YM
     , (sum(coalesce(z.NEW_MRR, 0))
            + sum(coalesce(z.EXPANSION_MRR, 0))
    - sum(coalesce(z.DOWNGRADE_MRR, 0))
    - sum(coalesce(z.CHURN_MRR, 0)))
                    MRR
from (select MONTHLY from MRR_DATA union all select date(max(monthly), '+1 month') as MONTHLY from MRR_DATA ) x
--      left join MRR_DATA y on date(x.MONTHLY, '-1 month') = date(y.MONTHLY)
     left join MRR_DATA z
on date (x.MONTHLY, '-1 month') >= date (z.MONTHLY)
group by x.monthly
order by 1 asc;