Is there a way to update groups of rows with separate incrementing values in one query - sql

Question

Lets say you have the following table:
Id Index
1 3
1 1
2 1
3 3
1 5
what I would like to have is the following:
Id Index
1 0
1 1
2 0
3 0
1 2
As you might notice, the goal is for every row where Id is the same, to incrementally update the Index column, starting from zero.
Now, I know this is fairly simple with using cursors, but out of curiosity is there a way to do this with single UPDATE query, somehow combining with temp tables, common table expressions or something similar?

Answer 1

Yes, assuming that the you don't really care about the order of the values for the new index values. SQL Server offers updatable CTEs and window functions that do exactly what you want:
with toupdate as (
select t.*, row_number() over (partition by id order by (select NULL)) as newindex
from table t
)
update toupdate
set index = newindex;
If you want them in a specific order, then you need another column to specify the ordering. The existing index column doesn't work.

Answer 2

With Row_number() -1 and CTE you can write as:
CREATE TABLE #temp1(
Id int,
[Index] int)
INSERT INTO #temp1 VALUES (1,3),(1,1),(2,1),(3,3),(1,5);
--select * from #temp1;
With CTE as
(
select t.*, row_number() over (partition by id order by (select null))-1 as newindex
from #temp1 t
)
Update CTE
set [Index] = newindex;
select * from #temp1;
Demo

Answer 3

I'm not sure why you would want to do this really, but I had fun figuring it out!
This solution relies on your table having a primary key for the self join... but you could always create an auto inc index if none exists and this is a one off job... This will also have the added benefit of getting you to think about the precise ordering of this you want... as currently there is no way of saying which order [ID] will get [Index] in.
UPDATE dbo.Example
SET [Index] = b.newIndex
FROM dbo.Example a
INNER JOIN (
select
z.ID,
z.[Index],
(row_number() over (partition by ID order by (select NULL))) as newIndex
from Example z
) b ON a.ID = b.ID AND a.[Index]=b.[Index] --Is this a unique self join for your table?.. no PK provided. You might need to make an index first.

Answer 4

Probably, this is what you want
SELECT *,RANK() OVER(PARTITION BY Id ORDER BY [Index])-1 AS NewIndex FROM
(
SELECT 1 AS Id,3 [Index]
UNION
SELECT 1,1
UNION
SELECT 2,1
UNION
SELECT 3,3
UNION
SELECT 1,5
) AS T
& the result will come as
Now if you want to update the table then execute this script
UPDATE tblname SET Index=RANK() OVER(PARTITION BY t.Id ORDER BY t.[Index])-1
FROM tblname AS t
In case I am missing something or any further assistance is required please let me know.

Answer 5

CREATE TABLE #temp1(
Id int,
Value int)
INSERT INTO #temp1 VALUES (1,2),(1,3),(2,3),(4,5)
SELECT
Id
,Value
,ROW_NUMBER() OVER (PARTITION BY Id ORDER BY Id) Id
FROM #temp1
Start with this :)
Gave me results like
Id Value Count
1 2 1
1 3 2
1 2 3
1 3 4
1 2 5
1 3 6
1 2 7
1 3 8
2 3 1
2 4 2
2 5 3
2 3 4
2 4 5
2 5 6
2 4 7
2 5 8
2 3 9
2 3 10
3 4 1
4 5 1
4 5 2
4 5 3
4 5 4