Potential Use of DENSE_RANK() in SQL

In SQL I'm looking for a potential function which I can use to apply a row number each time a certain criteria is met.

Each time the Unique ID is the same (i.e. same person) and the Dep_Group is WES then give that row the same number if they follow on from one another. What I'm trying to do is eventually squish those rows into one, hopefully by adding in a GROUP BY New_Col clause to the end. So in essence Line 1 & 2 become one line, also Line 5 & 6.

|Line| Uniq_ID | Start_DT      | End_DT        | Sequence | Department | Dep_Group | New_Col|
| 1  |   1     | 2023-01-02    | 2023-01-03    |    1     |     WES    |    WES    |     1  |
| 2  |   1     | 2023-01-02    | 2023-01-03    |    2     |     WES    |    WES    |     1  |
| 3  |   1     | 2023-01-02    | 2023-01-03    |    3     |     DEM    |    NULL   |     2  |
| 4  |   2     | 2023-01-02    | 2023-01-03    |    1     |     WES    |    WES    |     3  |
| 5  |   3     | 2023-01-02    | 2023-01-03    |    1     |     WES    |    WES    |     4  |
| 6  |   3     | 2023-01-02    | 2023-01-03    |    2     |     WES    |    WES    |     4  |
| 7  |   4     | 2023-01-02    | 2023-01-03    |    1     |     DEM    |    NULL   |     5  |

Example of the code I have so far which is not quite working as I wanted with DENSE_RANK():

SELECT Line
       ,Uniq_ID
       ,Start_DT
       ,End_DT
       ,Sequence
       ,Department
       ,Dep_Group
       ,DENSE_RANK() OVER (PARTITION BY Uniq_ID, Dep_Group ORDER BY Sequence) AS New_Col

FROM TblA

Solution

It is fairly straight forward: use running sum which increments if not (previous group = wes and current group = wes):

with cte1 as (
  select *
       , case when lag(dep_group) over (partition by uniq_id order by sequence) = 'WES' and dep_group = 'WES' then 0 else 1 end as incr
  from t
), cte2 as (
  select *
       , sum(incr) over (partition by uniq_id order by sequence) as grp_num
   from cte1
)
select *
from cte2

This gives you the following result:

line	uniq_id	start_dt	end_dt	sequence	department	dep_group	incr	grp_num
1	1	2023-01-02	2023-01-03	1	WES	WES	1	1
2	1	2023-01-02	2023-01-03	2	WES	WES	0	1
3	1	2023-01-02	2023-01-03	3	DEM	null	1	2
4	2	2023-01-02	2023-01-03	1	WES	WES	1	1
5	3	2023-01-02	2023-01-03	1	WES	WES	1	1
6	3	2023-01-02	2023-01-03	2	WES	WES	0	1
7	4	2023-01-02	2023-01-03	1	DEM	null	1	1

The grp_num column can be used to group rows along with uniq_id.