我想使用codeigniter显示这个结果

我有一个名为exam_time_table的表。我想得到每个类的时间表。我试图得到结果。但它没有得到。

i have a table named exam_time_table. i want to get the time table of each class. i tried to get the result.but its not getting.

控制器

public function index()
    {
       $this->load->model('exam_model','exam');
           $data['exam_data']=$this->exam->select_data();
           $this->load->view('welcome_message',$data);
    }

模型

public function select_data() {
        $this->db->distinct();
        $this->db->select('*');
        $this->db->from('exam_time_table');
        $this->db->group_by('extt_std');
        $query = $this->db->get();
return $query->result();
       }

查看

<?php
foreach($exam_data as $exam)
{
echo $exam->extt_id;
echo $exam->extt_sub;
echo $exam->extt_date;
}

?>