How Can I Dynamically Control the Number of Rows Returned by SQL Server's SELECT TOP?

Using dynamic variables in the SELECT TOP statement of SQL Server

In SQL Server, efficiently retrieving a specific amount of data is critical. However, using static values ​​in a SELECT TOP statement is inefficient when the number of required rows changes dynamically. This article introduces a method of using dynamic variables to control the number of rows returned.

Grammar and Usage

The following syntax demonstrates how to use variables to dynamically set the number of rows to return:

<code class="language-sql">DECLARE @count int;
SET @count = 20;

SELECT TOP (@count) * FROM SomeTable;</code>

Unlike the example provided in the question, this syntax includes parentheses around the variable name, which is critical for SQL Server 2005 and later.

Examples and explanations

Consider the following code:

<code class="language-sql">DECLARE @rowCount int;
SET @rowCount = 10;

SELECT TOP (@rowCount) *
FROM Customer
WHERE Age > 21
ORDER BY Age DESC;</code>

In this example, the DECLARE statement initializes the @rowCount variable to an integer. The subsequent SET statement assigns the value 10 to the variable. The SELECT statement uses the dynamic variable @rowCount in the TOP clause to retrieve the first 10 rows from the Customer table that satisfy the WHERE and ORDER BY conditions.

Compatibility

The method of using dynamic variables in SELECT TOP is compatible with SQL Server 2005 and later versions. Earlier versions do not support this syntax.

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