check for a column if it is null , over a previous term with some conditions - sql

Question

I have lets say two terms term A (previous)and term B (current) , i need to check if pol_cancl_date is null or not in term A , there is a transaction_sequence_number , i need to see if the pol_cncl_date is existing in the greatest A.transaction_sequence_number and if greatest( A.transaction_sequence_number ) is the greated when compared to all B.transaction_sequence_number numbers , if it is then i would want to check for pol_cancl_dates's existence and apply a logic
WITH x AS (
SELECT * FROM (
SELECT
pol_num
,term_start_dt
,term_end_dt,pol_cancel_dt
,trans_seq_num
,future_cancel_dt
,DENSE_RANK() OVER (PARTITION BY pol_num ORDER BY term_end_dt DESC) AS flag
FROM `gcp-ent-datalake-preprod.trns_prop_pol_hs_horison.prop_cost`
--WHERE pol_num IN ('30766675','33896642')
-- pol_num = '33288489'
ORDER BY term_start_dt, term_end_dt DESC
)
)
SELECT
*
,CASE
WHEN prior_pol_cancel_dt IS NOT NULL AND current_trans_seq_num < prior_trans_seq_num THEN prior_pol_cancel_dt
ELSE current_pol_cancel_dt
END apply_cancelled_renewal_dt
FROM (
SELECT
MAX(a.pol_num) AS current_pol_num
,MAX(a.term_start_dt) AS current_term_start_dt
,a.term_end_dt AS current_term_ent_dt
,MAX(a.pol_cancel_dt) AS current_pol_cancel_dt
,MAX(a.trans_seq_num) AS current_trans_seq_num
,MAX(a.future_cancel_dt) AS current_future_cancel_dt
,MAX(a.flag) AS current_flag
,MAX(b.pol_num) AS prior_pol_num
,MAX(b.term_start_dt) AS prior_term_start_dt
,b.term_end_dt AS prior_term_end_dt
,MAX(b.pol_cancel_dt) AS prior_pol_cancel_dt
,MAX(b.trans_seq_num) AS prior_trans_seq_num
,MAX(b.future_cancel_dt) AS prior_future_cancel_dt
,MAX(b.flag) AS prior_flag
FROM (
SELECT * FROM x WHERE flag=1) a
INNER JOIN(
SELECT * FROM x WHERE flag = 2 ) b
ON a.pol_num = b.pol_num AND a.flag = b.flag - 1
WHERE a.pol_cancel_dt IS NOT NULL
AND b.pol_cancel_dt IS NOT NULL
AND greatest(a.trans_seq_num) < b.trans_seq_num
-- AND a.trans_seq_num = GREATEST(a.trans_seq_num)
-- AND b.trans_seq_num = GREATEST(b.trans_seq_num)
GROUP BY a.term_end_dt, b.term_end_dt
)
--WHERE a.term_start_dt < b.term_start_dt
--if prior term GREATEST (trans_sewq num
this logic is still not giving me some results , one thing is that trans_seq_num doesn't necessarily have to be one less