void Wczytaj_klienci(JTable tab) {
try {
PreparedStatement pst = con.prepareStatement("Select * from Klienci");
ResultSet rs = pst.executeQuery();
            tab.setModel(DbUtils.resultSetToTableModel(rs));
} catch (Exception ex) {
            JOptionPane.showMessageDialog(null, ex.toString());
        }
 }