Have you ever needed to retrieve every Nth row from a result set in SQL Server? In this article, we will discuss a simple and dynamic T-SQL code that can accomplish this task without the need for loops, complex OFFSET-FETCH statements, or cursors.

The Problem

Let’s say you have a query and you want to retrieve every Nth row from the result set. For example, if N equals 2, you want to return every even row (second, fourth, sixth, etc.). The challenge is to create a T-SQL code that can handle this requirement dynamically.

The Solution

The solution involves creating a T-SQL stored procedure in SQL Server that takes an input SQL query string and the Nth parameter, and produces the expected result. Here is the T-SQL code for the stored procedure:

CREATE PROC usp_PickEveryNthRow (@qry VARCHAR (2000), @step INT)
AS
BEGIN
   DECLARE @CreateGlobalTemporaryTBTSQL VARCHAR (2000)
   DECLARE @FromClausePosition INT

   -- If objects ##result and/or table_sequence exist in the database, drop them.
   IF Object_Id('##RESULT') IS NOT NULL
      DROP TABLE ##RESULT

   IF Object_Id('table_sequence') IS NOT NULL
      DROP sequence table_sequence

   SET @FromClausePosition = CHARINDEX ('FROM', @qry, 1)
   -- Construct the global temporary table creation statement
   SET @CreateGlobalTemporaryTBTSQL = CONCAT (
         LEFT (@qry, @FromClausePosition - 1),
         ' INTO ##RESULT ',
         SUBSTRING (@qry, @FromClausePosition, LEN (@qry) - @FromClausePosition + 1)
         )

   EXEC (@CreateGlobalTemporaryTBTSQL)

   -- Add the running sequence to the global temporary table
   ALTER TABLE ##RESULT ADD table_sequence INT IDENTITY NOT NULL

   -- Retrieve every Nth row using the modulus operator in the WHERE clause
   SELECT *
   FROM ##RESULT
   WHERE table_sequence % @step = 0
   DROP TABLE ##RESULT
END
GO

Use Cases

The ability to retrieve every Nth row from a result set can be useful in various IT and business scenarios. It can be used for statistical applications, partitioning purposes, and more.

Limitations

There are a few limitations to using this stored procedure. Firstly, it always returns a running sequence in its result set. Additionally, it uses dynamic SQL execution, which is checked only upon execution and usually not cached in the SQL plan.

Performance Benefits

This stored procedure performs faster than using a loop, cursor, or complex OFFSET-FETCH statements. It uses a single SELECT statement from a global temporary table, filtering the rows using the modulus operator. The SELECT INTO statement used to fill the global temporary table does not use logging, making it faster than traditional methods.

Error Handling

If the input logic is incorrect, the stored procedure will fail during the dynamic execution statement. It is important to ensure that the query string is valid and that the step parameter is less than the number of returned rows in order to retrieve results successfully.

Example Usage

Let’s take a look at some examples using the stored procedure with the Northwind database:

Exec dbo.usp_PickEveryNthRow 'SELECT productname from products order by 1', 10

This example retrieves every tenth row from the products table, ordered by the product name.

Exec dbo.usp_PickEveryNthRow 'SELECT productname,unitprice from products where categoryid = 1 order by 2', 5

This example retrieves every fifth row from the products table for category 1, ordered by unit price.

By using this stored procedure, you can easily retrieve every Nth row from a result set without the need for complex code or performance-hindering methods. Give it a try and see how it can simplify your SQL Server queries!