Matching file in a directory to a filename in a SQL Server table

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C#

SQL-Server

In the code-behind (C#) of an aspx page the following
code retrieves all the files in a specified directory
and displays them in a dropdownlist. This portion of code works correctly:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs 
e)
    {
    }
    protected void Button1_Click(object sender, 
EventArgs e)
        
    {
        DirectoryInfo di = new DirectoryInfo(@"C:\Test");
        foreach (FileInfo f in di.GetFiles())
        {
            
            DropDownList1.Items.Add(f.Name);
        }
    }
}

I need some assistance with the following task:

Checking each filename in the directory to see
whether it exists in the SQL Server table tbl_ImportFileNames. If "f.name" does not exist in tbl_ImportFileNames.ImportFileName,
then I need to insert it into the table.

For example, if excelImport2003.xls exists as a filename in my directory, but not in the SQL Server table, I need to insert the filename into the SQL Server table.

This is the T-SQL (most likely needs correction) I have created in SQL Server ...

DECLARE @ImportFileName varchar(50)
set @ImportFileName = '??? f.name';

INSERT INTO tbl_ImportFileNames (ImportFileName)
SELECT ImportFileName
FROM tbl_ImportFileNames
WHERE NOT EXISTS (SELECT ImportFileName FROM 
tbl_ImportFileNames
WHERE @ImportFileName = 
tbl_ImportFileNames.ImportFileName);

How do I incorporate the query above into my aspx C# code-
behind? I would so appreciate any assistance!