$this->db->join(); is used to join tables in codeigniter.
Codeigniter join Multiple tables
$this->db->select('*');
$this->db->from('users');
$this->db->join('profile_image', 'profile_image.user_id = users.id');
$this->db->join('city', 'city.user_id = users.id','left');
$this->db->join('post', 'post.user_id = users.id','left');
$this->db->join('friends', 'friends.user_id = users.id','left');
$this->db->where('users.id', $id);
$query = $this->db->get();
Will produce
// Select *from users
join profile_image on profile_image.user_id = users.id
left join city on city.user_id = users.id
left join post on post.user_id = users.id
left join friends on friends.user_id = users.id
where users.id = $id