Set value for group by order

Question

I have SQL table with columns COL_1, COL_2:

COL_1	COL_2	COL_3
a	2021-09-11 14:25:15.699	1
b	2021-09-11 14:25:36.489	2
a	2021-09-11 14:33:36.500	3
a	2021-09-11 14:36:24.915	3
a	2021-09-11 14:36:38.900	3
a	2021-09-11 14:36:54.905	3
c	2021-09-11 14:37:09.092	4
a	2021-09-11 14:39:09.111	5
d	2021-09-11 14:39:30.292	6

I need to get COL_3. I got column 4 by grouping column 1 and sorting by date (column 2), if the group changes, I assign a new index for the following groups.

Answer 1

This is a classic gaps-and-islands problem.

There are many solutions. A common one is to use LAG to identify rows which change value, then a conditional windowed COUNT to assign the group numbers

WITH PrevValues AS (
    SELECT *,
      LAG(COL_1) OVER (ORDER BY COL_2) AS PrevCol1
    FROM YourTable t
)
SELECT COL_1,COL_2,COL_3,
  COUNT(CASE WHEN PrevCol1 = COL_1 THEN NULL ELSE 1 END)
      OVER (ORDER BY COL_2 ROWS UNBOUNDED PRECEDING) AS New_Col_3
FROM PrevValues t

db<>fiddle