select
    YM
    , sum(TMP_MRR) over(order by YM rows between unbounded preceding and current row) as MRR
from (select
        MONTHLY as YM
        , coalesce(lag(NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR, 1)
            over(order by MONTHLY), 0) as TMP_MRR
    from MRR_DATA)
order by YM
;