Understanding the N’’ Prefix in Stored Procedures
When working with stored procedures, one common issue developers face is the addition of a prefix to parameters, such as N'' or single quotes. In this article, we’ll explore why this happens and how it can be resolved.
The Problem at Hand
The question comes from a developer who’s experiencing an error when executing a stored procedure in SQL Server. They’re passing four arguments: startdate, enddate, coursecode, and subjectcode. After running the stored procedure through the SQL Server Management Studio (SSMS) GUI, they notice that some prefixes have been added to their parameters.
Here’s the relevant code snippet:
EXEC @return_value = [dbo].[GET_ATTENDANCE_REPORT_FOR_FACULTY]
@startdate = '1/9/2018',
@enddate = '1/12/2018',
@coursecode = N'''BSCCS''',
@subjectcode = N'''CSHT101'''
And here’s the stored procedure itself:
CREATE PROCEDURE GET_ATTENDANCE_REPORT_FOR_FACULTY
@startdate DATE,
@enddate DATE,
@coursecode varchar(10),
@subjectcode varchar(10)
AS BEGIN
-- Procedure code here
The issue arises when they try to remove the N'' prefix from their parameters in C# code. For example:
cmd.Parameters.AddWithValue("@coursecode", courseCode);
Instead, they get an error saying “Invalid column name ‘BSCCS’”.
The Cause of the Problem
In SQL Server, when you use dynamic SQL or string concatenation to build a query, the server executes the parameters as columns. This is because SQL Server assumes that any unquoted parameter is a column.
The problem lies in this line:
SELECT RollNo,FirstName,LastName, ' + dbo.fn_convert_cols(@cols) + ' from
(
select S.RollNo,U.FirstName,U.LastName,
D.startdate,
convert(CHAR(6), startdate, 106) PivotDate
from #tempDates D,Attendance A, Student S, UserDetails U
where convert(CHAR(6), D.startdate, 106) = convert(CHAR(6), A.Date, 106) and A.EnrollmentNo=S.EnrollmentNo and A.EnrollmentNo=U.userID and A.CourseCode=''' + @coursecode + ''' and A.SubjectCode =''' + @subjectcode + '''
) x
pivot
(
count(startdate)
for PivotDate in (' + @cols + ')
) p
When you use a parameter like @coursecode without quotes, SQL Server assumes it’s a column name. In this case, the column name is not defined anywhere else in the procedure.
To fix this issue, we need to ensure that any unquoted parameter is treated as literal text, rather than a column.
The Solution
The solution lies in modifying the dynamic query to properly quote the parameters:
SET @query = 'SELECT RollNo,FirstName,LastName, ' + dbo.fn_convert_cols(@cols) + ' from
(
select S.RollNo,U.FirstName,U.LastName,
D.startdate,
convert(CHAR(6), startdate, 106) PivotDate
from #tempDates D,Attendance A, Student S, UserDetails U
where convert(CHAR(6), D.startdate, 106) = convert(CHAR(6), A.Date, 106) and A.EnrollmentNo=S.EnrollmentNo and A.EnrollmentNo=U.userID and A.CourseCode=''' + @coursecode + ''' and A.SubjectCode =''' + @subjectcode + '''
) x
pivot
(
count(startdate)
for PivotDate in (' + @cols + ')
) p '
By adding double quotes around @coursecode and @subjectcode, we ensure that SQL Server treats them as literal text, rather than column names.
This change will prevent the addition of prefixes to your parameters, making it easier to work with dynamic queries in stored procedures.
Last modified on 2024-03-11