Db2 Developer-C 11.1

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WITH TAB (GRP_ID, YR_MO, MONTHLY_COST, NEXT_RENEWAL_YR_MO) AS 
(
VALUES 
  ('001', '201202', 10, '202007')
, ('001', '201201', 10, '202007')
, ('001', '201203', 10, '202007')
, ('001', '201204', 10, '202007')
, ('001', '201205', 10, '202007')
, ('001', '201206', 10, '202007')
, ('001', '201207', 10, '202007')
, ('001', '201208', 10, '202007')
, ('001', '201209', 10, '202007')
, ('001', '201210', 10, '202007')
, ('001', '201211', 10, '202007')
, ('001', '201212', 10, '202007')
, ('001', '201301', 10, '202007')
, ('001', '201302', 10, '202007')
, ('001', '201303', 10, '202007')
, ('001', '201304', 10, '202007')
, ('001', '201305', 10, '202007')
, ('001', '201306', 10, '202007')
, ('001', '201307', 10, '202007')
, ('001', '201308', 10, '202007')
)
SELECT 
  GRP_ID, YR_MO
, SUM(CASE WHEN RIGHT(YR_MO, 2) = RIGHT(NEXT_RENEWAL_YR_MO, 2) THEN 1 ELSE 0 END) OVER (PARTITION BY GRP_ID ORDER BY YR_MO) + 1 AS POLICY_PERIOD
, MONTHLY_COST
, SUM(MONTHLY_COST) OVER 
(
  PARTITION BY 
  GRP_ID
, SUM(CASE WHEN RIGHT(YR_MO, 2) = RIGHT(NEXT_RENEWAL_YR_MO, 2) THEN 1 ELSE 0 END) OVER (PARTITION BY GRP_ID ORDER BY YR_MO)
  ORDER BY YR_MO
) AS RUN_TOT_AMT_PER_PLCY_PRD
FROM TAB;

GRP_ID	YR_MO	POLICY_PERIOD	MONTHLY_COST	RUN_TOT_AMT_PER_PLCY_PRD
001	201201	1	10	10
001	201202	1	10	20
001	201203	1	10	30
001	201204	1	10	40
001	201205	1	10	50
001	201206	1	10	60
001	201207	2	10	10
001	201208	2	10	20
001	201209	2	10	30
001	201210	2	10	40
001	201211	2	10	50
001	201212	2	10	60
001	201301	2	10	70
001	201302	2	10	80
001	201303	2	10	90
001	201304	2	10	100
001	201305	2	10	110
001	201306	2	10	120
001	201307	3	10	10
001	201308	3	10	20