Redshift AWS - Update table with lag() in sub query and cte - sql-update

Question

I have a Redshift database with the following entries:
table name = subscribers
time_at
calc_subscribers
calc_unsubscribers
current_subscribers
2021-07-02 07:30:00
0
0
0
2021-07-02 07:45:00
39
8
0
2021-07-02 08:00:00
69
17
0
2021-07-02 08:15:00
67
21
0
2021-07-02 08:30:00
48
23
0
The goal is to calculate current_subscribers with the previous value.
current_subscribers = calc_subscribers - calc_unsubscribers + previous_current_subscribers
I do the following:
UPDATE subscribers sa
SET current_subscribers = COALESCE( sa.calc_subscribers - sa.calc_unsubscribers + sub.previous_current_subscribers,0)
FROM (
SELECT
time_at,
LAG(current_subscribers, 1) OVER
(ORDER BY time_at desc) previous_current_subscribers
FROM subscribers
) sub
WHERE sa.time_at = sub.time_at
The problem is that in the sub query "sub" a table is generated that is based on the current values in the table, and thus previous_current_subscribers is always 0. Instead of going through this row by row. So the result is: current_subscribers = calc_subscribers - calc_unsubscribers + 0 I have also already tried it with CTE, unfortunately without success:
The result should look like this:
time_at
calc_subscribers
calc_unsubscribers
current_subscribers
2021-07-02 07:30:00
0
0
0
2021-07-02 07:45:00
39
8
31
2021-07-02 08:00:00
69
17
83
2021-07-02 08:15:00
67
21
129
2021-07-02 08:30:00
48
95
82
I am grateful for any ideas.

Answer 1

The problem you are running into is that you want to use the result of one row in the calculation of the current row. This is recursive which I think you can do in this case but is expensive.
The result you are looking for is the sum of all calc_subscribers for this row and previous rows minus the sum of all calc_unsubscribers for this row and previous rows. This is the difference between 2 window functions - sum over.
sum(calc_subscribers) over (order by time_at desc rows unbounded preceding) - sum(calc_unsubscribers) over (order by time_at desc rows unbounded preceding) as current_subscribers