如何访问只为管理员登录页面

我希望只有在人员表中角色为A（A=管理员）的人员才能访问管理员模块。我希望谭只能访问登录，而其他员工不能访问。

这是输出：

点击图像

数据库：

点击数据库表

这是我的代码：

 SqlConnection conn = null;
        SqlCommand cmd = null;
        string connectionString = null;
        string staffName = null;
        protected void Page_Load(object sender, EventArgs e)
        {
        }
        public bool CheckValidUser(string Username, string Password)
        {
            bool valid = false;
            SqlDataReader dr = null;
            connectionString = ConfigurationManager.ConnectionStrings["LeaveManagementCS"].ConnectionString;
            string sql = "SELECT * from Staff WHERE Username=@Username AND Password=@Pwd";
            try
            {
                conn = new SqlConnection(connectionString);
                cmd = new SqlCommand(sql, conn);
                cmd.Parameters.AddWithValue("@Username", Username);
                cmd.Parameters.AddWithValue("@Pwd", Password);
                conn.Open();
                dr = cmd.ExecuteReader();
                if (dr.Read())
                {
                    staffName = dr["StaffName"].ToString();
                    valid = true;
                }
                else
                {
                    lblOutput.Text = "There is an error logging in. Please check username or password.";
                }
                dr.Close();
            }
            catch (Exception ex)
            {
                lblOutput.Text = "Error Message: " + ex.Message;
            }
            finally
            {
                if (conn != null)
                    conn.Close();
            }
            return valid;
        }
        protected void tbLogin_Click(object sender, EventArgs e)
        {
            bool validUser = CheckValidUser(tbUsername.Text, tbPassword.Text);
            if (validUser)
            {
                Session["StaffName"] = staffName;
                FormsAuthentication.SetAuthCookie(staffName, false);
                FormsAuthentication.RedirectFromLoginPage(staffName, false);
            }
            else
            {
                lblOutput.Text = "Invalid User. Please try again.";
            }
        }

string sql = "SELECT * from Staff WHERE Username=@Username AND Password=@Pwd And Role=N'A';