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

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();