codeigniter - SQL查询'IN'运算符不起作用错误？

我正在使用CI'in'operator无法正常运行sql错误，请检查其并分享宝贵的想法...

表

在此处输入图片说明

id   | coach_name
------------------
9    | GS
------------------
10   | SLR 

视图和功能

$coachID = explode(',',$list['coach']);
$coachname = $this->rail_ceil_model->display_coach_name($coachID); 

显示结果

单反相机

需要结果

GS，单反

上一个查询结果

coach_name从mcc_coach哪里选择id（'9'，'10'）

CI代码

public function display_coach_name($coachID='')
            {
        $db2 = $this->load->database('rail',TRUE);
        $db2->select('coach_name');
        $db2->from('mcc_coach');
        $db2->where_in('id',$coachID);
        $query = $db2->get();
        echo $db2->last_query(); die;

        if ($query->num_rows() > 0):
            //return $query->row()->coach_name;
        else:
            return 0;
        endif;
            }