select
    x.monthly YM,
    sum(ifnull(a.g, 0)) over (order by x.monthly) MRR
from
    (select monthly from mrr_data union all select DATE(max(monthly), '+1 months') from mrr_data) x
    left join (
        select
            MONTHLY,
            NEW_MRR + EXPANSION_MRR - DOWNGRADE_MRR - CHURN_MRR as g
        from
            MRR_DATA
    ) A on x.MONTHLY = DATE(A.MONTHLY, '+1 months')
order by
    x.monthly