left join ไม่ได้ครับ ก่อน left join โหลดข้อมูลได้ปกติ

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.OleDb;
using System.IO;

namespace project1
{   
    
    public partial class frmSearch : Form
    {
        OleDbDataAdapter oda;
        OleDbCommandBuilder ocb;
        DataTable dt;
        public frmSearch()
        {
            InitializeComponent();
        }

        private void btn_Search_Click(object sender, EventArgs e)
        {
            OleDbConnection conn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\Student.mdb");
            DataTable dt = new DataTable();
            OleDbDataAdapter oda = new OleDbDataAdapter("SELECT tb_Student.Behaviour,tb_test.Test FROM tb_Student LEFT JOIN tb_test ON tb_Student.Fname = tb_test.Fname WHERE tb_Student.Fname = '" + this.txtSearch.Text + "'", conn);
            oda.Fill(dt);
            dataGridSearch.DataSource = dt;
        }
    }
}

private void btn_Search_Click(object sender, EventArgs e)
        {
            OleDbConnection conn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=\\Student.mdb");
            DataTable dt = new DataTable();
            OleDbDataAdapter oda = new OleDbDataAdapter("SELECT Behaviour,Day FROM tb_Student WHERE Fname = '" + this.txtSearch.Text + "'", conn);
            oda.Fill(dt);
            dataGridSearch.DataSource = dt;
        }