How to make one column match duplicates in another column - sql

Question

This problem is out of my ability range and I can’t get anywhere with it beyond knowing I can probably use LEAD, LAG or maybe a cursor?
Here is a breakdown of the table and question:
row_id is always an IDENTITY(1, 1) column.
The set_id column always starts out in groups of 3s (two 0s for the first set_id, don't worry about why).
The letter column is alphabetic. There are varying counts of duplicates.
Here's the original table:
row_id
set_id
letter
1
0
A
2
0
A
3
1
A
4
1
B
5
1
B
6
2
B
7
2
B
8
2
C
9
3
C
10
3
C
11
3
D
12
4
D
13
4
D
14
4
D
What I need is a code that: if there is a duplicate letter in the next row, then the set_id in the next row should be the same as the previous row (alt_set_id).
If that doesn't make sense, here is the result I want:
row_id
set_id
letter
alt_set_id
1
0
A
0
2
0
A
0
3
1
A
0
4
1
B
1
5
1
B
1
6
2
B
1
7
2
B
1
8
2
C
2
9
3
C
2
10
3
C
2
11
3
D
3
12
4
D
3
13
4
D
3
14
4
D
3
Here's where I am with code so far, I'm not really close but I think I am on the right path:
SELECT
*,
CASE
WHEN letter = [letter in next row]
THEN 'yes'
ELSE 'no'
END AS 'next row a duplicate?',
'tbd' AS alt_row_id
FROM
(SELECT
*,
LEAD(letter) OVER (ORDER BY row_id) AS 'letter in next row'
FROM
sort_test) AS dt
WHERE
row_id = row_id
That query has the below result set, which is something I think I can work with, but it doesn't feel very efficient and I'm not yet getting the result needed in the alt_set_id column:
row_id
set_id
letter
letter in next row
next row a duplicate?
alt_set_id
1
0
A
A
yes
tbd
2
0
A
A
yes
tbd
3
1
A
B
no
tbd
4
1
B
B
yes
tbd
5
1
B
B
yes
tbd
6
2
B
B
yes
tbd
7
2
B
C
no
tbd
8
2
C
C
yes
tbd
9
3
C
C
yes
tbd
10
3
C
D
no
tbd
11
3
D
D
yes
tbd
12
4
D
D
yes
tbd
13
4
D
D
yes
tbd
14
4
D
NULL
no
tbd
Thanks for any help!

Answer 1

Based on your example data, you want the minimum set_id for each letter. If so, use window functions;
select t.*, min(set_id) over (partition by letter) as alt_set_id
from sort_test t;

Answer 2

It would appear if I understand correctly a simple correlated subquery will give you the desired result:
select *, (select Min(set_Id) from t t2 where t2.letter=t.letter) as alt_set_id
from t
See working DB Fiddle