Get top 1 row of each group

Question

I have a table which I want to get the latest entry for each group. Here's the table:

DocumentStatusLogs Table

ID	DocumentID	Status	DateCreated
2	1	S1	7/29/2011
3	1	S2	7/30/2011
6	1	S1	8/02/2011
1	2	S1	7/28/2011
4	2	S2	7/30/2011
5	2	S3	8/01/2011
6	3	S1	8/02/2011

The table will be grouped by DocumentID and sorted by DateCreated in descending order. For each DocumentID, I want to get the latest status.

My preferred output:

DocumentID	Status	DateCreated
1	S1	8/02/2011
2	S3	8/01/2011
3	S1	8/02/2011

• Is there any aggregate function to get only the top from each group? See pseudo-code GetOnlyTheTop below:

    SELECT
      DocumentID,
      GetOnlyTheTop(Status),
      GetOnlyTheTop(DateCreated)
    FROM DocumentStatusLogs
    GROUP BY DocumentID
    ORDER BY DateCreated DESC
  

• If such function doesn't exist, is there any way I can achieve the output I want?

• Or at the first place, could this be caused by unnormalized database? I'm thinking, since what I'm looking for is just one row, should that status also be located in the parent table?

Please see the parent table for more information:

Current Documents Table

DocumentID	Title	Content	DateCreated
1	TitleA	...	...
2	TitleB	...	...
3	TitleC	...	...

Should the parent table be like this so that I can easily access its status?

DocumentID	Title	Content	DateCreated	CurrentStatus
1	TitleA	...	...	s1
2	TitleB	...	...	s3
3	TitleC	...	...	s1

UPDATE I just learned how to use "apply" which makes it easier to address such problems.

Answer 1

WITH cte AS
(
   SELECT *,
         ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC) AS rn
   FROM DocumentStatusLogs
)
SELECT *
FROM cte
WHERE rn = 1

If you expect 2 entries per day, then this will arbitrarily pick one. To get both entries for a day, use DENSE_RANK instead of ROW_NUMBER.

As for normalised or not, it depends if you want to:

• maintain status in 2 places
• preserve status history
• ...

As it stands, you preserve status history. If you want latest status in the parent table too (which is denormalisation) you'd need a trigger to maintain "status" in the parent. or drop this status history table.