with months as (
select monthly from (select date(monthly, '+1 month') as monthly from mrr_data union all select monthly from mrr_data) group by monthly order by monthly
)
, m1 as (select monthly as YM
, ifnull((select new_mrr + expansion_mrr - downgrade_mrr - churn_mrr from mrr_data where mrr_data.monthly = date(m1.monthly, '-1 month')), 0) as pre_mmr
from months as m1
)
select YM, sum(pre_mmr) over (order by YM) as MRR from m1