How to increment in a select query

Question

I've got a query I'm working on and I want to increment one of the fields and restart the counter when a key value is different.

I know this code doesn't work. Programmatically this is what I want...

declare @counter int, @id
set @counter = 0
set @id = 0

select distinct 
  id, 
  counter = when id = @id 
              then @counter += 1
            else @id = id  
               @counter = 1     

...with the end result looking something like this:

ID    Counter
3     1
3     2 
3     3
3     4
6     1
6     2
6     3
7     1

And yes, I am stuck with SQL2k. Otherwise that row_number() would work.

Answer 1

Yes you want ROW_NUMBER().

I would try: SELECT id, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY ID) AS Counter

Answer 2

Assuming a table:

CREATE TABLE [SomeTable] (
  [id] INTEGER,
  [order] INTEGER,
  PRIMARY KEY ([id], [order])
);

One way to get this in Microsoft SQL Server 2000 is to use a subquery to count the rows with the same id and a lower ordering.

SELECT *, (SELECT COUNT(*) FROM [SomeTable] counter 
           WHERE t.id = counter.id AND t.order < counter.order) AS row_num
FROM [SomeTable] t

Tip: It's 2010. Soon your SQL Server will be old enough to drive.

If you use SQL Server 2005 or later, you get wonderful new functions like ROW_NUMBER() OVER (PARTITION...).

Answer 3

Having row_number() means you have to deal with far, far fewer correlated subqueries. @Bill Karwin's solution works (+1); here's another version that does the same thing but that might be a bit easier to follow. (I used datetimes to determine ordering.)

--  Test table
CREATE TABLE Test
 ( Id      int       not null
  ,Loaded  datetime  not null
 )

--  Load dummy data with made-up distinct datetimes
INSERT Test values (3, 'Jan 1, 2010')
INSERT Test values (3, 'Jan 2, 2010')
INSERT Test values (3, 'Jan 5, 2010')
INSERT Test values (3, 'Jan 7, 2010')
INSERT Test values (6, 'Feb 1, 2010')
INSERT Test values (6, 'Feb 11, 2010')
INSERT Test values (7, 'Mar 31, 2010')


--  The query
SELECT t1.Id, count(*) Counter
 from Test t1
  inner join Test t2
   on t2.Id = t1.Id
    and t2.Loaded <= t1.Loaded
 group by t1.Id, t1.Loaded


--  Clean up when done
DROP TABLE Test

It is important to note that, without good indexes (and perhaps even with them), these kinds of queries can perform very poorly, particularly on large tables. Check and optimize carefully!

Answer 4

One way to do this is to throw the data into a temp table with an identity column that is used as a row number. Then make the counter column a count of the other rows with the same Id and a lower row number + 1.

CREATE TABLE #MyData(
Id INT
);

INSERT INTO #MyData VALUES(3);
INSERT INTO #MyData VALUES(3);
INSERT INTO #MyData VALUES(3);
INSERT INTO #MyData VALUES(3);
INSERT INTO #MyData VALUES(6);
INSERT INTO #MyData VALUES(6);
INSERT INTO #MyData VALUES(6);
INSERT INTO #MyData VALUES(7);

CREATE TABLE #MyTempTable(
RowNum INT IDENTITY(1,1),
Id INT,
Counter INT
);

INSERT INTO #MyTempTable
SELECT Id, 0
FROM #MyData
ORDER BY Id;

SELECT Id, (SELECT COUNT(*) + 1 FROM #MyTempTable WHERE Id = t1.Id AND RowNum < t1.RowNum) AS 'Counter'
FROM #MyTempTable t1;

You should get the following output based on your example:

Id  Counter
3   1
3   2
3   3
3   4
6   1
6   2
6   3
7   1