Why Does My SQL Stored Procedure Throw a 'Must Declare the Scalar Variable' Error?

Unveiling the Mystery of "Must Declare the Scalar Variable"

In the realm of SQL Stored Procedures, attempting to utilize global input parameters like "@RowFrom" and "@RowTo" within dynamically generated SQL statements can often lead to the perplexing error, "Must declare the scalar variable."

This issue arises because the parameters are not explicitly declared within the SQL string (@sqlstatement) used to compile the query. To rectify this, it is necessary to either:

Utilize CONVERT() to Convert Variables to Strings:

Instead of concatenating an int-type variable directly, use CONVERT() to transform it into a string. For instance:

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

This ensures that the value of "@RowTo" can be assigned to "@Rt" as a string.

Employ Modern CONCAT() Function:

In newer SQL versions, the CONCAT() function simplifies parameter concatenation, eliminating the need for manual conversion.

SET @sql = CONCAT(N'SELECT ', @RowTo, ' * 5');

Adopt Secure Parameterization:

However, it is highly recommended to opt for proper parameterization rather than string concatenation. This approach minimizes the risk of SQL injection by separating SQL code from user-input parameters.

SET @sql = @sql + ' WHERE RowNum BETWEEN @RowFrom AND @RowTo;';

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

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