How to Get the Top 1 Row of Each Group in SQL

Sql Sql Server T Sql Sql Server 2005 Greatest N Per Group

When working with large datasets in SQL, it is often necessary to retrieve only the top row from each group based on certain criteria. This can be a common requirement when dealing with log tables or transactional data. In this article, we will explore different approaches to solve this problem in SQL and provide code examples for SQL Server.

Understanding the Problem

Let's start by understanding the problem at hand. We have a table called 'DocumentStatusLogs' which contains information about the status of different documents over time. The table has the following columns: ID, DocumentID, Status, and DateCreated.

            
                | 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   |

Our goal is to retrieve the latest entry for each document, based on the date it was created. We want the result to include the DocumentID, Status, and DateCreated columns, like this:

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

Approach 1: Using Subqueries

One way to solve this problem is by using subqueries. We can write a query that selects the top row for each DocumentID based on the DateCreated column. Here's the code:

            
                SELECT
                    DocumentID,
                    Status,
                    DateCreated
                FROM
                    DocumentStatusLogs l
                WHERE
                    DateCreated = (
                        SELECT
                            MAX(DateCreated)
                        FROM
                            DocumentStatusLogs
                        WHERE
                            DocumentID = l.DocumentID
                    )

In the above query, we use a subquery to select the maximum DateCreated value for each DocumentID. We then use this value to filter the rows in the outer query, effectively selecting only the top row for each group.

Approach 2: Using Window Functions

Another approach to solving this problem is by using window functions, specifically the ROW_NUMBER() function. This function assigns a unique number to each row within a partition, based on the specified order. We can then filter the rows based on this row number to get the top rows.

            
                SELECT
                    DocumentID,
                    Status,
                    DateCreated
                FROM (
                    SELECT
                        DocumentID,
                        Status,
                        DateCreated,
                        ROW_NUMBER() OVER (PARTITION BY DocumentID ORDER BY DateCreated DESC) as rn
                    FROM
                        DocumentStatusLogs
                ) l
                WHERE
                    rn = 1

In the above query, we use the ROW_NUMBER() function to assign a unique row number for each row within a partition (in this case, the partition is the DocumentID). We then filter the rows where the row number is equal to 1, effectively selecting only the top row for each group.

Conclusion

In this article, we explored different approaches to solving the problem of retrieving the top 1 row from each group in SQL. We covered two approaches: using subqueries and using window functions. Both approaches are valid and can be used depending on the specific requirements and database system being used.

Remember to choose the approach that best suits your needs and optimize the query for performance if working with large datasets. It is always recommended to test the queries on a subset of data before running them on the entire dataset.

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