Select that returns only new status changes - sql

Question

I would like to select all the accounts that status changed to B1 during the month of December 2020. That is, the accounts that now have B1 status and in the previous record did not have.
Records with VALID_TO_DATE = 31/12/2100 (DD/MM/YYYY) are the most recent records.
However, there are cases, such as account 400, that the change in status occurred in the penultimate record, which is no longer valid. But as the change happened in December and the status is the same as the valid record, my select statement must include account 400.
Desired result: In this sense, according to the data below, I need a select that returns only accont 100 and 400
ROW
ACCOUNT_ID
ACCOUNT_CHANGE_DATE
VALID_TO_DATE
ACCOUNT_STATUS
1
100
21/10/2020
05/12/2020
A1
2
100
05/12/2020
15/12/2020
A1
3
100
15/12/2020
31/12/2100
B1
4
200
09/11/2020
22/12/2020
A1
5
200
22/12/2020
25/12/2020
A1
6
200
25/12/2020
31/12/2100
A1
7
300
11/11/2020
19/11/2020
B1
8
300
19/11/2020
23/12/2020
A1
9
300
23/12/2020
31/12/2100
A1
10
400
16/11/2020
14/12/2020
A1
11
400
14/12/2020
18/12/2020
B1
12
400
18/12/2020
31/12/2100
B1
My source table has records of all changes that affected each of the accounts, which may or may not change the status of the respective account.
This is my incomplete/wrong attempt:
SELECT ACCOUNT_ID
FROM TABLE t1
where ACCOUNT_STATUS ="B1"
AND ACCOUNT_CHANGE_DATE BETWEEN '01/12/2020' AND '31/12/2020'
AND VALID_TO_DATE = '31/12/2100'
AND NOT EXISTS (SELECT * FROM TABLE t2
WHERE t2.ACCOUNT_ID = t1.ACCOUNT_ID
AND t2.ACCOUNT_STATUS = 'B1'
AND t2.ACCOUNT_CHANGE_DATE < t1.ACCOUNT_CHANGE_DATE);

Answer 1

I would like to select all the accounts that status changed to B1 during the month of December 2020.
Use lag():
select t1.*
from (select t1.*,
lag(status) over (partition by account_id order by ACCOUNT_CHANGE_DATE) as prev_status
from t1
) t1
where prev_status <> 'B1' and status = 'B1' and
account_change_date >= '2021-12-01' and
account_change_date < '2022-01-01'