Biguqery Row Number Perioding based on specific values - sql

Question

So i have a raw data like this.
| User_id | Month | Device
| 001 | 01 | A
| 001 | 02 | A
| 001 | 03 | B (Base)
| 001 | 04 | A
| 002 | 01 | C
| 002 | 02 | C
| 002 | 03 | B (Base)
| 002 | 04 | B (Base)
| 003 | 01 | A
| 003 | 02 | B (Base)
| 003 | 03 | A
| 003 | 04 | B (Base)
I want to create period that split by B (Base) Device. I thinking about using Row Number but breaks only if meet B or several B's.
The result i want will be like this
| User_id | Month | Device | rn
| 001 | 01 | A | 1
| 001 | 02 | A | 1
| 001 | 03 | B (Base)| 2
| 001 | 04 | A | 3
| 002 | 01 | C | 1
| 002 | 02 | C | 1
| 002 | 03 | B (Base)| 2
| 002 | 04 | B (Base)| 2
| 003 | 01 | A | 1
| 003 | 02 | B (Base)| 2
| 003 | 03 | A | 3
| 003 | 04 | B (Base)| 4
Is there any way we can populate rn like that?

Answer 1

You might consider below query.
SELECT * EXCEPT(flag), COUNTIF(flag) OVER w + 1 AS rn FROM (
SELECT *,
(LAG(Device) OVER w <> 'B' AND Device = 'B') OR
(LAG(Device) OVER w = 'B' AND Device <> 'B') AS flag
FROM sample_table
WINDOW w AS (PARTITION BY User_id ORDER BY Month)
) WINDOW w AS (PARTITION BY User_id ORDER BY Month)
ORDER BY User_id, Month;
+---------+-------+--------+----+
| User_id | Month | Device | rn |
+---------+-------+--------+----+
| 001 | 01 | A | 1 |
| 001 | 02 | A | 1 |
| 001 | 03 | B | 2 |
| 001 | 04 | A | 3 |
| 002 | 01 | C | 1 |
| 002 | 02 | C | 1 |
| 002 | 03 | B | 2 |
| 002 | 04 | B | 2 |
| 003 | 01 | A | 1 |
| 003 | 02 | B | 2 |
| 003 | 03 | A | 3 |
| 003 | 04 | B | 4 |
+---------+-------+--------+----+