with S as (
    select
        L.MONTHLY as YM,
        ifnull(R.NEW_MRR + R.EXPANSION_MRR - R.DOWNGRADE_MRR - R.CHURN_MRR, 0) as MMR
    from
        MRR_DATA L
    left outer join
        MRR_DATA R
    on
        date(L.MONTHLY, '-1 months') = R.MONTHLY
)

select
    YM,
    sum(MMR) over (order by YM) as MRR
from
    S
order by
    YM asc