Derive and Update Column Value based on Row Value SQL Server - sql

Question

So I have a Request History table that I would like to flag its versions (version is based on end of cycle); I was able to mark the end of the cycle, but somehow I couldn't update the values of each associated with each cycle. Here is an example:
|history_id | Req_id | StatID | Time |EndCycleDate |
|-------------|---------|-------|---------- |-------------|
|1 | 1 |18 | 3/26/2017 | NULL |
|2 | 1 | 19 | 3/26/2017 | NULL |
|3 | 1 |20 | 3/30/2017 | NULL |
|4 |1 | 23 |3/30/2017 | NULL |
|5 | 1 |35 |3/30/2017 | 3/30/2017 |
|6 | 1 |33 |4/4/2017 | NULL |
|7 | 1 |34 |4/4/2017 | NULL |
|8 | 1 |39 |4/4/2017 | NULL |
|9 | 1 |35 |4/4/2017 | 4/4/2017 |
|10 | 1 |33 |4/5/2017 | NULL |
|11 | 1 |34 |4/6/2017 | NULL |
|12 | 1 |39 |4/6/2017 | NULL |
|13 | 1 |35 |4/7/2017 | 4/7/2017 |
|14 | 1 |33 |4/8/2017 | NULL |
|15 | 1 | 34 |4/8/2017 | NULL |
|16 | 2 |18 |3/28/2017 | NULL |
|17 | 2 |26 |3/28/2017 | NULL |
|18 | 2 |20 |3/30/2017 | NULL |
|19 | 2 |23 |3/30/2017 | NULL |
|20 | 2 |35 |3/30/2017 | 3/30/2017 |
|21 | 2 |33 |4/12/2017 | NULL |
|22 | 2 |34 |4/12/2017 | NULL |
|23 | 2 |38 |4/13/2017 | NULL |
Now what I would like to achieve is to derive a new column, namely VER, and update its value like the following:
|history_id | Req_id | StatID | Time |EndCycleDate | VER |
|-------------|---------|-------|---------- |-------------|------|
|1 | 1 |18 | 3/26/2017 | NULL | 1 |
|2 | 1 | 19 | 3/26/2017 | NULL | 1 |
|3 | 1 |20 | 3/30/2017 | NULL | 1 |
|4 |1 | 23 |3/30/2017 | NULL | 1 |
|5 | 1 |35 |3/30/2017 | 3/30/2017 | 1 |
|6 | 1 |33 |4/4/2017 | NULL | 2 |
|7 | 1 |34 |4/4/2017 | NULL | 2 |
|8 | 1 |39 |4/4/2017 | NULL | 2 |
|9 | 1 |35 |4/4/2017 | 4/4/2017 | 2 |
|10 | 1 |33 |4/5/2017 | NULL | 3 |
|11 | 1 |34 |4/6/2017 | NULL | 3 |
|12 | 1 |39 |4/6/2017 | NULL | 3 |
|13 | 1 |35 |4/7/2017 | 4/7/2017 | 3 |
|14 | 1 |33 |4/8/2017 | NULL | 4 |
|15 | 1 | 34 |4/8/2017 | NULL | 4 |
|16 | 2 |18 |3/28/2017 | NULL | 1 |
|17 | 2 |26 |3/28/2017 | NULL | 1 |
|18 | 2 |20 |3/30/2017 | NULL | 1 |
|19 | 2 |23 |3/30/2017 | NULL | 1 |
|20 | 2 |35 |3/30/2017 | 3/30/2017 | 1 |
|21 | 2 |33 |4/12/2017 | NULL | 2 |
|22 | 2 |34 |4/12/2017 | NULL | 2 |
|23 | 2 |38 |4/13/2017 | NULL | 2 |

Answer 1

One method that comes really close is a cumulative count:
select t.*,
count(endCycleDate) over (partition by req_id order by history_id) as ver
from t;
However, this doesn't get the value when the endCycle date is defined exactly right. And the value starts at 0. Most of these problems are fixed with a windowing clause:
select t.*,
(count(endCycleDate) over (partition by req_id
order by history_id
rows between unbounded preceding and 1 preceding) + 1
) as ver
from t;
But that misses the value on the first row first one. So, here is a method that actually works. It enumerates the values backward and then subtracts from the total to get the versions in ascending order:
select t.*,
(1 + count(*) over (partition by req_id) -
(count(endCycleDate) over (partition by req_id
order by history_id desc)
) as ver
from t;