void countCustomers() {
        DateFormat df = new SimpleDateFormat("yyyy-MM-dd");
        Date d = new Date();
        String currentDate = df.format(d);
        String sql = "SELECT COUNT(*) FROM Sales WHERE SUBSTR(DateTime, 1, 10) = ?;";
        String count = getPersist().read(String.class, sql, currentDate);
        lblCustomers.setText(String.format("来客数 %s 人", count));
    }