what i write inside loop to display similar item code on datagridview and insert different itemcode on database sql server

Problem

SQL Server Database(2014) Items Table

ItemCode(pk)    ItemName  

001                       mouse

002                       keyboard

003                       Headphone

On File Excel sheet 2010     

ItemCode              ItemName  

001                           mouse

002                           keyboard

004                           screen

005                           Ram

Actually i need when import excel file insert different items code that not exist

 on sql server database and Exist Items On Database and Found on Excel not insert but display on datagridview .

according to my case insert itemcodes 004,005 on table Items.

and show 001,002 in grid view as exist items . 

my function as below 

my code (Inside Loop)

public static void ImportFromExcelToDataBase()
    {
       Datatable dt = ShowExcelData();
       DataTable dtItems = GetSqlItems();
            for (int i = 0; i < dt.Rows.Count; i++)
            {
                //what i write here 
                // if itemcode exist on excel exist on sql server database
                then display similar items exist on database and excel as 001,002 on datagridview
                //else 
                 // do insert data
                string Insert = "Insert Into Values (" + data + ")";
                DataAccess.ExecuteNonQuery(Insert);
            }
    }
 public DataTable ShowExcelData()
        {
            string connectionString = string.Format("Provider=Microsoft.ACE.OLEDB.12.0;Data Source={0};Extended Properties=\"Excel 12.0 Xml;HDR=YES;IMEX=1\";", txtpath.Text);

            OleDbConnection con = new OleDbConnection(connectionString);


            con.Open();
            DataTable dt = new DataTable();

            dt = con.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);

            string SheetName = dt.Rows[0]["TABLE_NAME"].ToString();


            OleDbCommand com = new OleDbCommand();
            com.Connection = con;
           
            com.CommandText = @"SELECT  [ItemCode],[ItemsName],[ItemAddress] FROM  [" + SheetName + "] ";
            OleDbDataAdapter oledbda = new OleDbDataAdapter();
            oledbda.SelectCommand = com;
            DataSet ds = new DataSet();
            oledbda.Fill(ds);
            dt = ds.Tables[0];
            con.Close();
            return dt;


        }
dt = ShowExcelData();

 public DataTable GetSqlItems()
        {
            string GetItems = @"select ItemCode,ItemsName,ItemAddress from Items";


           DataTable tbGetItems = DataAccess.ExecuteDataTable(GetItems );
            return tbGetItems ;
        }
dtItems = GetSqlItems();