Используйте это для более безопасных запросов:
$query_username = $this->db->query("SELECT COUNT(username) AS count_username FROM users WHERE username=?", $this->input->post('username'));
$row_username = $query_username->row_array();
if ($row_username['count_username'] > 0) {
$query_password = $this->db->query("SELECT password FROM users WHERE username=?", $this->input->post('username'));
$row_password = $query_password->row_array();
if ($row_password['password'] == MD5($this->input->post('password')) {
// LOGIN SUCCESS
} else {
// LOGIN FAILED
}
} else {
// LOGIN FAILED
}