遇到 PHP 错误 严重性: 注意 消息: 未定义的变量: id 文件名: 型号/singlequery_model.p

A PHP Error was encountered
Severity: Notice
Message: Undefined variable: id
Filename: models/singlequery_model.php
Line Number: 4
A Database Error Occurred
Error Number: 1064
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1

选择 * 从患者，其中 ID =

文件名： C：''xampp''htdocs''login''system''database''DB_driver.php

行号：330

这是查询单个结果的正确方法吗？

    <?php
class Singlequery_model extends CI_Model{
    function get_query(){
        $query = $this->db->query("SELECT * FROM patient WHERE id  = $id");
        if($query->num_rows() > 0 ){
            foreach ($query->result() as $row){
                $data[] = $row;
            }
            return $data;
        }
    }
}

下面是我的控制器

    <?php 
class Site extends CI_Controller{
    function __construct(){
      parent::__construct();
      $this->is_logged_in();
      $this->load->model('site_model');
    }
    function delete(){
      $this->site_model->delete_row();
      redirect('site/members_area');
    }
    function view(){
        $this->load->model('singlequery_model');
        $data['records'] = $this->singlequery_model->get_query();
        $this->load->view('patient_update', $data);
    }

}