Retrieving Top Records by Category

In many database applications, it’s common to need to retrieve a limited number of top records for each category or group. This can be achieved using various SQL techniques. In this tutorial, we’ll explore how to write efficient queries to get the top 10 records for each category.

To solve this problem, we’ll use the ROW_NUMBER() and RANK() window functions, which are available in most modern databases, including SQL Server, MySQL, and PostgreSQL. These functions allow us to assign a unique number or rank to each row within a partition of a result set.

Understanding Window Functions

Before diving into the query examples, let’s briefly discuss how window functions work. A window function performs calculations across a set of rows that are related to the current row, such as rows with the same category or group. The PARTITION BY clause divides the result set into partitions based on one or more columns.

The ROW_NUMBER() and RANK() functions assign a unique number or rank to each row within a partition. The main difference between these two functions is how they handle ties:

• ROW_NUMBER(): Assigns a unique number to each row, even if there are ties.
• RANK(): Assigns the same rank to tied rows and skips the next rank value.

Retrieving Top Records by Category

Now that we understand window functions, let’s write a query to retrieve the top 10 records for each category. We’ll use the following table structure as an example:

CREATE TABLE Records (
    RecordID INT,
    Section VARCHAR(50),
    DateEntered DATETIME,
    Field1 VARCHAR(100),
    Field2 VARCHAR(100)
);

To get the top 10 records for each section, we can use the ROW_NUMBER() function with a subquery:

SELECT r.RecordID, r.Section, r.DateEntered, r.Field1, r.Field2
FROM (
    SELECT RecordID, Section, DateEntered, Field1, Field2,
        ROW_NUMBER() OVER (PARTITION BY Section ORDER BY DateEntered DESC) AS RowNo
    FROM Records
) r
WHERE r.RowNo <= 10;

In this query:

• The subquery uses ROW_NUMBER() to assign a unique number to each row within each section, ordered by the DateEntered column in descending order.
• The outer query selects only the rows with a RowNo value less than or equal to 10.

Alternatively, we can use the RANK() function:

SELECT r.RecordID, r.Section, r.DateEntered, r.Field1, r.Field2
FROM (
    SELECT RecordID, Section, DateEntered, Field1, Field2,
        RANK() OVER (PARTITION BY Section ORDER BY DateEntered DESC) AS RankValue
    FROM Records
) r
WHERE r.RankValue <= 10;

Note that the RANK() function may return more than 10 rows if there are ties in the ranking.

Common Table Expressions (CTEs)

Another approach to solving this problem is by using a common table expression (CTE). A CTE is a temporary result set that we can reference within a query. Here’s how we can use a CTE to get the top 10 records for each section:

WITH TopRecords AS (
    SELECT RecordID, Section, DateEntered, Field1, Field2,
        ROW_NUMBER() OVER (PARTITION BY Section ORDER BY DateEntered DESC) AS RowNo
    FROM Records
)
SELECT * FROM TopRecords WHERE RowNo <= 10;

In this query:

• The CTE (TopRecords) uses ROW_NUMBER() to assign a unique number to each row within each section.
• The main query selects only the rows from the CTE with a RowNo value less than or equal to 10.

Conclusion

Retrieving top records by category is a common task in database applications. By using window functions like ROW_NUMBER() and RANK(), we can efficiently get the top records for each group. Whether you prefer subqueries, CTEs, or other approaches, understanding how window functions work will help you write more efficient queries.