在我的网站上，用户可以选择注册，然后登录，但我想知道我该怎么做，以便用户可以注销后。
c代码：这是登录的代码。

protected void Page_Load(object sender, EventArgs e)
        {
            lb_Fail.Visible = false;
        }

        protected void btn_Login_Click(object sender, EventArgs e)

        {
            // connecion to the database
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["Fasthosts_MMConnectionString"].ConnectionString);

            SqlDataAdapter sda = new SqlDataAdapter("SELECT COUNT(*) FROM Fasthosts_Registration_MM WHERE username='" + tb_UsernameLogin.Text + "' AND password='" + tb_PasswordLogin.Text + "'", conn);
            /* in above line the program is selecting the whole data from table and the matching it with the user name and password provided by user. */
            DataTable dt = new DataTable(); //this is creating a virtual table  
            sda.Fill(dt);
            if (dt.Rows[0][0].ToString() == "1")
            {

                Response.Redirect("Domains.aspx");

            }
            else
            {
                lb_Fail.Text = ("Incorrect login details");
                lb_Fail.Visible = true;
            }
        }

如果创建了注销按钮，我需要什么代码。