Why Does My T-SQL Stored Procedure Throw a 'Must Declare the Scalar Variable' Error, and How Can I Fix It?

Must Declare the Scalar Variable: Resolving the Error

When executing a SQL query inside a stored procedure using T-SQL, it's common to encounter the error "Must declare the scalar variable" when using global input parameters such as @RowFrom and @RowTo. This issue arises when the procedure attempts to utilize these parameters within the @sqlstatement variable.

Understanding the Error

The error occurs because T-SQL expects all scalar variables, including those defined as input parameters, to be explicitly declared before they can be used. In the given scenario, @RowFrom and @RowTo are not declared within the @sqlstatement variable, leading to the error.

Solution: Declaring Parameters

To resolve this error, we need to declare the scalar variables within the @sqlstatement variable. This can be achieved using the DECLARE statement, as shown in the modified code below:

SET @sql = N'DECLARE @Rt int; SET @Rt = ' + CONVERT(VARCHAR(12), @RowTo);

In this code, we declare the scalar variable @Rt and assign the value of @RowTo to it after converting it to a string using the CONVERT function. This ensures that @Rt is properly declared and can be used within the @sqlstatement variable.

Concat vs. Parameterization

While concatenation can be used to build the SQL string, it's generally recommended to use parameterization for security reasons. By using parameters, we minimize the risk of SQL injection attacks. The modified code below illustrates how parameterization can be used:

SET @sql = N'SELECT @RowFrom, @RowTo;';

EXEC sys.sp_executesql @sql,
  N'@RowFrom int, @RowTo int',
  @RowFrom, @RowTo;

By using parameterization, we provide the values of @RowFrom and @RowTo to the stored procedure without concatenating them into the SQL statement, ensuring the security of our code.

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