How can I create a row id within a set of rows - sql-server

Question

I have some data like
Id, GroupId, Whatever
1, 1, 10
2, 1, 10
3, 1, 10
4, 2, 10
5, 2, 10
6, 3, 10
And I need to add a "group row id" column such as
Id, GroupId, Whatever, GroupRowId
1, 1, 10 1
2, 1, 10 2
3, 1, 10 3
4, 2, 10 1
5, 2, 10 2
6, 3, 10 1
Ideally it would be computed and enforced by the database. So when I do
INSERT INTO Foos (GroupId, Whatever) VALUES (1, 20)
I'd get the correct GroupRowId. Continuing the example data above, this row would then look like
Id, GroupId, Whatever, GroupRowId
7, 1, 20 4
This data is to be shared with a 3rd party and one of the requirements is for those GroupRowIds to be fixed regardless of any different ORDER BY or WHERE clauses.
I've considered a view with a row_id over/partition by but that view could still be modified in the future breaking previously shared data.
Our business rules dictate that no rows will be deleted so the GroupRowId will never need to be recomputed in this respect and there will never** be missing values.
** in the perfect world of business rules.
My thinking is that it would be preferable that this be a physical column so that it exists within the row. It can be queried and won't change based on a ORDER BY or WHERE clause.

Answer 1

You might try something along this:
--create a test database (will be dropped at the end! Carefull with real data!!)
USE master;
GO
CREATE DATABASE GroupingTest;
GO
USE GroupingTest;
GO
--Your table, I use an IDENTITY column for your Id column
CREATE TABLE dbo.tbl(Id INT IDENTITY,GroupId INT,Whatever INT);
GO
--Insert your test values
INSERT INTO tbl(GroupId, Whatever)
VALUES
(1,10)
,(1,10)
,(1,10)
,(2,10)
,(2,10)
,(3,10);
GO
--This is necessary to add the new column and to fill it initially
ALTER TABLE tbl ADD GroupRowId INT;
GO
WITH cte AS
(
SELECT GroupRowId
,ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY Id) AS NewValue
FROM tbl
)
UPDATE cte SET GroupRowId=NewValue;
--check the result
SELECT * FROM tbl ORDER BY GroupId,Id;
GO
--Now we create a trigger, which does exactly the same for new rows
--Very important: This must work with single inserts and with multiple inserts as well!
CREATE TRIGGER dbo.SetNextGroupRowId ON dbo.tbl
FOR INSERT
AS
BEGIN
WITH cte AS
(
SELECT GroupRowId
,ROW_NUMBER() OVER(PARTITION BY GroupId ORDER BY Id) AS NewValue
FROM tbl
)
UPDATE cte
SET GroupRowId=NewValue
WHERE GroupRowId IS NULL; --<-- this ensures to change only new rows
END
GO
--Now we can test this with a single value
INSERT INTO tbl(GroupId, Whatever)
VALUES(1,20);
SELECT * FROM tbl ORDER BY GroupId,Id;
--And we can test this with multiple inserts
INSERT INTO tbl(GroupId, Whatever)
VALUES
(1,30)
,(2,30)
,(2,30)
,(3,30)
,(4,30); --<-- the "4" is a new group
SELECT * FROM tbl ORDER BY GroupId,Id;
GO
--Cleaning
USE master;
GO
DROP DATABASE GroupingTest;
What you should keep in mind:
This might get in troubles with values inserted manually into GroupRowId or with any manipulation of this column by any other statement.
This might get in troubles with deleted rows
You can think about an approach selecting MAX(GroupRowId)+1 for the given group. This depends on your needs.
You might add an unique index on GroupId,GroupRowId. This would - at least - avoid giving the same number twice, but would lead into an error.
...but in your perfect world of business rules :-) this won't happen...
And to be honest: The whole issue has some smell...