Retrieve the Top N Rows for Each Category SQL Server

Retrieve the Top N Rows for Each Category Query Using MS SQL Server?

To retrieve the top N rows for each category in SQL Server, you can use window functions, specifically ROW_NUMBER(), which allows you to rank rows within each category. Then, you can filter based on this ranking.

Here’s a general approach assuming you have a table with categories and some criteria to rank the rows (e.g., sales, scores, etc.):

Example Table Structure

Assume you have a table:

• Products:
  • ProductID
  • CategoryID
  • ProductName
  • SalesAmount

SQL Query to Retrieve the Top N Rows for Each Category

WITH RankedProducts AS (

    SELECT

        ProductID,

        CategoryID,

        ProductName,

        SalesAmount,

        ROW_NUMBER() OVER (PARTITION BY CategoryID ORDER BY SalesAmount DESC) AS rn

    FROM Products

)

SELECT

    ProductID,

    CategoryID,

    ProductName,

    SalesAmount

FROM RankedProducts

WHERE rn <= @TopN

ORDER BY CategoryID, rn;

Explanation:

1. CTE RankedProducts:

   • ROW_NUMBER() OVER (PARTITION BY CategoryID ORDER BY SalesAmount DESC) AS rn: This assigns a unique sequential integer (rn) to each row within each CategoryID partition, ordered by SalesAmount in descending order. This means the highest sales amount gets 1, the next gets 2, and so on.

2. Final Query:

   • WHERE rn <= @TopN: Filters the rows to include only those where the row number is less than or equal to @TopN, which is a parameter representing the top N rows you want to retrieve for each category.

Example Data and Results

Assume the Products table has the following data:

ProductID	CategoryID	ProductName	SalesAmount
1	A	Product 1	1000
2	A	Product 2	1500
3	A	Product 3	2000
4	B	Product 4	3000
5	B	Product 5	3500
6	B	Product 6	2500
7	C	Product 7	500
8	C	Product 8	700

If you want to retrieve the top 2 products per category, setting @TopN to 2, the query will result in:

ProductID	CategoryID	ProductName	SalesAmount
3	A	Product 3	2000
2	A	Product 2	1500
5	B	Product 5	3500
4	B	Product 4	3000
8	C	Product 8	700
7	C	Product 7	500

This result shows the top 2 products by sales amount for each category.

You can execute this query in SQL Server Management Studio or any SQL query tool connected to your SQL Server to retrieve the top N rows for each category. Adjust the @TopN parameter to the number of top rows you need.