Retain values only for certain section of data in teradata - teradata

Question

Below is the link of my previous quetsion.
Retain values till there is a change in value in Teradata
It worked as suggested by one of the community members #Dnoeth. Can this retention be done only for certain section of data?
I.e, Retain data only for data where Dep is A or B . When Dep is C just use same value as input and no need to retain till certain value.
Data:
Cust_id Balance st_ts Dep
123 1000 27MAY2018 A
123 350 31MAY2018 A
256 2000 29MAY2018 B
345 1000 28APR2018 C
345 1200 26MAY2018 C
Output reqd:
Cust_id Balance st_ts Dep
123 1000 27MAY2018 A
123 1000 28MAY2018 A
123 1000 29MAY2018 A
123 1000 30MAY2018 A
123 350 31MAY2018 A
256 2000 29MAY2018 B
256 2000 30MAY2018 B
256 2000 31MAY2018 B
345 1000 28APR2018 C
345 1200 26MAY2018 C
Query used:
Wth cte
{
SELECT customer_id, bal, st_ts,
-- return the next row's date
Coalesce(Min(st_ts)
Over (PARTITION BY customer_id
ORDER BY st_ts
ROWS BETWEEN 1 Following AND 1 Following)
,Date '2018-06-01') AS next_Txn_dt
FROM BAL_DET;
}
SELECT customer_id, bal
,Last(pd) -- last day of the period
FROM cTE
-- make a period of the current and next row's date
-- and return one row per day
EXPAND ON PERIOD(ST_TS, next_Txn_dt) AS pd;
Thanks
Sandy

Answer 1

You can add a CASE to check for Dep = 'C':
WITH cte AS
( SELECT customer_id, bal, st_ts, dep,
-- return the next row's date
CASE
WHEN dep = 'C'
THEN st_ts +1 -- simply increase date
ELSE
Coalesce(Min(st_ts)
Over (PARTITION BY customer_id
ORDER BY st_ts
ROWS BETWEEN 1 Following AND 1 Following)
,DATE '2018-06-01')
END AS next_Txn_dt
FROM BAL_DET
)
SELECT customer_id, bal
,Last(pd) -- last day of the period
,dep
FROM cTE
-- make a period of the current and next row's date
-- and return one row per day
EXPAND ON PERIOD(ST_TS, next_Txn_dt) AS pd