DataGridView控件操作数据库只更新控件上的信息不更新数据库的问题

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.SqlClient;

namespace 实验
{
public partial class Form1 : Form
{
static string connString = @"Data Source=DESKTOP-SFFEEKC\SQLEXPRESS;Initial Catalog=学生信息表;Integrated Security=SSPI";
SqlConnection conn = new SqlConnection(connString);

    public Form1()
    {
        InitializeComponent();
    }
    private void button1_Click(object sender, EventArgs e)
    {
        SqlDataAdapter da = new SqlDataAdapter("SELECT * FROM Table_2 where 班级='" + comboBox1.Text + "'", conn);
        DataTable dt = new DataTable();
        SqlCommandBuilder cmd = new SqlCommandBuilder(da);
        da.Update(dt);
        MessageBox.Show("更新成功");
        conn.Close();
    }

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        conn.Open();
        string strQuery = "SELECT * FROM Table_2 where 班级='" + comboBox1.Text + "'";
        SqlCommand command = new SqlCommand(strQuery, conn);
        SqlDataAdapter da = new SqlDataAdapter(strQuery, conn);
        DataTable dt = new DataTable();
        da.Fill(dt);
        dataGridView1.DataSource = dt;
        conn.Close();
    }
}
    }
    大神们帮我看看，为什么我更新了后，只改变了控件上的数据，并没有改变数据库的信息，我想通过这个控件该数据库里的数据

http://blog.csdn.net/qq_23833037/article/details/52167239