SELECT 
M1.MONTHLY AS YM
,CASE WHEN M3.MONTHLY IS NULL AND M2.MONTHLY IS NULL THEN 0
      WHEN M3.MONTHLY IS NULL AND M2.MONTHLY IS NOT NULL THEN (M2.NEW_MRR + M2.EXPANSION_MRR - M2.DOWNGRADE_MRR - M2.CHURN_MRR)
      ELSE (M3.NEW_MRR + M3.EXPANSION_MRR - M3.DOWNGRADE_MRR - M3.CHURN_MRR) + (M2.NEW_MRR + M2.EXPANSION_MRR - M2.DOWNGRADE_MRR - M2.CHURN_MRR) 
 END AS MRR
FROM MRR_DATA M1
LEFT JOIN MRR_DATA M2
ON (DATETIME(M1.MONTHLY,'-1 month') = DATETIME(M2.MONTHLY) )
LEFT JOIN MRR_DATA M3
ON (DATETIME(M1.MONTHLY,'-2 month') = DATETIME(M3.MONTHLY) )
ORDER BY M1.MONTHLY