SQL reset row_number when previous id column null

Question

This is hard to explain so I will give an example. I need SQL (ms server), I assume its with row_number over partition but can't get it to work.

I have this table:

ID	PreviousID	Data
1		a
2	1	b
3	2	c
4		d
5	4	e
6		f

I want these results:

ID	NewID	Data
1	1	a
2	1	b
3	1	c
4	2	d
5	2	e
6	3	f

And another with just the new IDs of each sequence:

NewID	Data
1	a
2	d
3	f

Instead of a row number new id, it could also have the first id of the sequence, whatever is easier, as long as it identifies the sequence.

Answer 1

Seems you want a windowed COUNT of rows where the value of PreviousID is NULL.

SELECT ID,
       COUNT(CASE WHEN PreviousID IS NULL THEN 1 END) OVER (ORDER BY ID) AS NewID,
       Data
FROM dbo.YourTable;