By: Eli Leiba | Comments (7) | Related: > Testing
Problem
The requirement is to create simple and dynamic T-SQL code in SQL Server that will get a query string and return every Nth row from the result set of that query. For example, if N equals 2 then the procedure should return every even row (second, fourth, sixth, etc.). Both the query and the N should be set dynamically in the T-SQL code. Another requirement mainly for performance purposes is to avoid loops, complex OFFSET-FETCH statements or cursors. How can we accomplish this?
Solution
The solution involves creating a T-SQL stored procedure in SQL Server that gets an input SQL query string and the Nth parameter then produces the expected result.
- The procedure gets a valid query @qry and @step parameters
- The procedure checks whether the objects called ##RESULT and table_sequence exists and if so it drops them.
- The procedure creates a global temporary table ##RESULT by constructing a SELECT INTO statement from the given query and executes it dynamically.
- The procedure adds the table_sequence column to the newly created temporary table as a not null identity column and by that forcing it to count all rows from 1 to the number of rows in the temporary table.
- The results of the procedure are all the rows where the modulus of the table_sequence column and the @step parameter equals zero.
Here is the T-SQL code for the stored procedure
-- =====================================================
-- Author: Eli Leiba
-- Create date: 11-2017
-- Description: Get every nth row from a given query
-- =====================================================
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 exists in Database then 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
-- pull every Nth row by using the modulus operator in the where clause
SELECT *
FROM ##RESULT
WHERE table_sequence % @step = 0
DROP TABLE ##RESULT
END
GO
Keep in mind the following with this code:
- What are the use cases for using the stored procedure from an IT and business
perspective?
- The ability to pick every Nth row of a given result set can be used for statistical applications, partitioning purposes and more.
- What are the limitations to using this stored procedure?
- The limitations of using this procedure are that it always returns a running sequence in its result set.
- It uses dynamic SQL execution that is checked only upon execution and usually the SQL plan is not in cache.
- How does this stored procedure perform faster than a loop, cursor, etc.?
- The procedure uses a single SELECT statement from a global temporary table filtering the rows by using the modulus operator with a SELECT INTO statement that does not use logging to fill the global temporary table.
- These are simple SQL statements that by default are faster that using loops and cursors in T-SQL code.
- What happens to the stored procedure if the input logic is wrong?
- The input parameters for this procedure are a query string an integer numbered as the step N for each Nth row.
- The query string that is supposed to be a valid query. If the string contains an invalid query then the procedure will fail in the dynamic execution statement.
- The integer number should be less than the number of returned rows in order to return results.
Here are some examples for using the procedure
Let’s take a look at some examples from Microsoft's demonstration database - NORTHWIND.
Get every tenth row of the products table ordered by the product name
Exec dbo.usp_PickEveryNthRow 'SELECT productname from products order by 1', 10
Product name table_sequence =============================== ============== Chef Anton's Cajun Seasoning 10 Grandma's Boysenberry Spread 20 Jack's New England Clam Chowder 30 Mishi Kobe Niku 40 Queso Cabrales 50 Scottish Longbreads 60 Tofu 70
Get every fifth row of the products table for category 1 ordered by unit price
Exec dbo.usp_PickEveryNthRow 'SELECT productname,unitprice from products where categoryid = 1 order by 2',5
And the result is:
productname unitprice table_sequence ============ ========= ============== Cate de Blaye 263.50 5 Rhanbrau Klosterbier 7.75 10
Next Steps
- You can create this simple procedure in your application database and use it for similar functionality in your applications queries.
- The input query is assumed to be a valid simple SELECT query and validity should be checked prior to executing this procedure.
- The procedure uses a table called ##RESULT and a sequence called table_sequence, so don't use these names in your database or update the script.
- The procedures were tested for SQL Server version:
- Microsoft SQL Server 2014 - 12.0.2000.8 (Intel X86)
- Feb 20 2014 19:20:46
- Copyright (c) Microsoft Corporation
- Standard Edition on Windows NT 6.1 <X86> (Build 7601: Service Pack 1)
- The stored procedure should be compatible with SQL Server 2012 version and above.
About the author
Eli Leiba is a senior application DBA, a teacher and a senior database consultant with 19 years of RDBMS experience.This author pledges the content of this article is based on professional experience and not AI generated.
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