为什么单击登录按钮时出现这种错误。


为什么单击登录按钮时  “在da.Fill(ds, "yhb");处显示FROM子句语法错误。”  出现这种错误。private void button1_Click(object sender, EventArgs e)        {
            string strUser = txtUser.Text.ToString().Trim();           
            string strPwd ="";
            strPwd = txtPassword.Text.ToString().Trim();            OleDbConnection conn = new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=mybook.mdb");
            conn.Open();
            DataSet ds = new DataSet();
            string sql = "select * from user";
            OleDbDataAdapter da = new OleDbDataAdapter(sql, conn);
            da.Fill(ds, "yhb");
            DataTable dt = new DataTable();
            dt = ds.Tables["yhb"];            for (int i = 0; i < dt.Rows.Count; i++)
            {
                if (strUser == dt.Rows[i]["username"].ToString().Trim())
                {
                    if (strPwd == dt.Rows[i]["password"].ToString().Trim())
                    {
                        this.DialogResult = DialogResult.OK;
                    }
                    else
                    {
                        MessageBox.Show("你的用户名或密码不正确,请重新输入!");
                        return;
                    }
                }        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            LoginForm  fm = new LoginForm ();
            if (fm.ShowDialog() == DialogResult.OK)
            {
                Application.Run(new MainForm ());
            }
            LoginForm form1 = new LoginForm();
            form1.Close();
            MainForm form2 = new MainForm();
            form2.Show();
        }
                
      }