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 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;