Retrieving Top Records from Each Group in SQL

In this tutorial, we will explore how to retrieve the top record from each group in a database table using SQL. This is a common problem that arises when working with grouped data and wanting to extract only the most relevant or latest information.

Understanding the Problem

Let’s consider an example where we have a table called DocumentStatusLogs that stores the status history of documents. The table has columns for ID, DocumentID, Status, and DateCreated. We want to retrieve the latest status for each document, grouped by DocumentID.

Approaches to Solve the Problem

There are several approaches to solve this problem, including:

1. Using ROW_NUMBER() or DENSE_RANK(): These window functions allow us to assign a ranking to each row within a group based on a specified order.
2. Using CROSS APPLY: This operator allows us to apply a subquery to each row of the main query, effectively retrieving the top record for each group.
3. Using TOP 1 WITH TIES: This clause allows us to retrieve the top record from each group, with ties being handled accordingly.

Using ROW_NUMBER() or DENSE_RANK()

Here is an example using ROW_NUMBER():

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

This query assigns a ranking to each row within each group based on the DateCreated column in descending order. The top record for each group is then retrieved by filtering on rn = 1.

Using CROSS APPLY

Here is an example using CROSS APPLY:

SELECT d.DocumentID, ds.Status, ds.DateCreated 
FROM Documents AS d 
CROSS APPLY (
  SELECT TOP 1 Status, DateCreated
  FROM DocumentStatusLogs 
  WHERE DocumentID = d.DocumentID
  ORDER BY DateCreated DESC
) AS ds;

This query applies a subquery to each row of the Documents table, retrieving the top record for each group based on the DateCreated column.

Using TOP 1 WITH TIES

Here is an example using TOP 1 WITH TIES:

SELECT TOP 1 WITH TIES
   DocumentID,
   Status,
   DateCreated
FROM DocumentStatusLogs
ORDER BY ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC);

This query retrieves the top record from each group, with ties being handled accordingly.

Performance Considerations

When choosing an approach, it’s essential to consider performance. The CROSS APPLY method is often the most efficient, especially for large datasets. However, the best approach will depend on the specific database system and data distribution.

Normalization Considerations

Finally, it’s worth considering whether the data should be normalized or denormalized. In this example, we have a separate table for document status history. If we want to maintain the latest status in the parent table as well, we would need to consider denormalizing the data and using triggers or other mechanisms to maintain consistency.

In conclusion, retrieving top records from each group is a common problem that can be solved using various SQL approaches. By understanding the problem and considering performance and normalization factors, you can choose the best approach for your specific use case.