DatabaseOldWay.php

<?php
  
// CodeIgniter coding style with plain queries, easy to read

$users = $this->db->query('SELECT * FROM Users')->result();

$cities = $this->db->query('SELECT * FROM Cities')->result();

$usersWithCity = $this->db->query('SELECT U.*, C.city_name FROM Users U LEFT JOIN Cities C ON U.id_user = C.id_user')->result();

$messages = $this->db->query('SELECT * FROM Messages ORDER BY date_sent');

/*
 *  Using LEFT JOIN in server-side web development is a good practice (IMHO) and here's why :
 *  
 * Consider tables A and B
 *  
 *  Using LEFT JOIN will KEEP A's records even if they don't have any reference to B ones
 *  Using INNER JOIN will SKIP A's records not having a reference to B ones
 *
 *  However, you'd like to be able to show A's data with a special clause like :
 *  show_A(); if ( there_is_nothing_from_B() ) show_empty_message();
 *  
 *   A common scenario is instant-messaging apps, you want the user profile and its messages
 */

?>