Function to detect change in an ordered list - sql

Question

I who like to find a solution in T-SQL that could find a a way to detect a change in a given list or records.
The physical table is like this:
| id |val |
|----|----|
| 1 | A |
|----|----|
| 2 | A |
|----|----|
| 3 | B |
|----|----|
| 4 | B |
|----|----|
| 5 | A |
|----|----|
| 6 | A |
|----|----|
id is a sequencial integer
val is an arbitrary value
I would like to add an calculated field that could somehow denote a change of val
Desired result:
| id |val | segment |
|----|----|---------|
| 1 | A | 1 |
|----|----|---------|
| 2 | A | 1 |
|----|----|---------|
| 3 | B | 2 |
|----|----|---------|
| 4 | B | 2 |
|----|----|---------|
| 5 | A | 3 |
|----|----|---------|
| 6 | A | 3 |
|----|----|---------|
What I'm trying to do is the possibility to group by "segments" like this:
| from_id | to_id | val |
|---------|-------|-----|
| 1 | 2 | A |
| 3 | 4 | B |
| 5 | 6 | A |
|---------|-------|-----|

Answer 1

Assuming SQL Server 2005+
DECLARE #T TABLE (
id INT PRIMARY KEY,
val CHAR(1))
INSERT INTO #T
SELECT 1,'A' UNION ALL SELECT 2,'A' UNION ALL
SELECT 3,'B' UNION ALL SELECT 4,'B' UNION ALL
SELECT 5,'A' UNION ALL SELECT 6,'A'
;WITH cte1 AS(
SELECT
id,
val,
ROW_NUMBER() OVER (ORDER BY id) - ROW_NUMBER() OVER (PARTITION BY val ORDER BY id) AS Grp
FROM #T
),
cte2 AS(
SELECT
id,
val,
MIN(id) OVER (PARTITION BY Grp, val) AS GrpStart
FROM cte1
)
SELECT
id,
val,
DENSE_RANK() OVER (ORDER BY GrpStart) AS segment
FROM cte2
Or the updated requirement is a bit simpler
;WITH cte AS(
SELECT
id,
val,
ROW_NUMBER() OVER (ORDER BY id) - ROW_NUMBER() OVER (PARTITION BY val ORDER BY id) AS Grp
FROM #T
)
SELECT
val,
MIN(id) AS from_id,
MAX(id) AS to_id
FROM cte
GROUP BY Grp, val
ORDER BY from_id

Answer 2

This might have to be modified a bit to work. Also, i never use loops, do this might not be the most efficient way to do it.
Declare #counter int
Set #counter = 1
Declare #seg int
Set #seg = 1
Declare #cur varchar(50)
Set # cur = select val from table where id = 1
While #counter <= select max id from table
Begin
if(#cur == select val from table where id = #counter)
update table set segment = #seg where id = #counter
else
{
set #cur = select val from table where id = #counter
set # seg = #seg + 1
update table set segment = #seg where id = #counter
}
Set #counter = #counter + 1
End
Well thats the general idea anyways...