判断表是否存在的问题 [ 2.0 版本 ]

我是想判断表是否存在，然后添加表
但提示 Call to a member function createCommand() on a non-object
请问要怎么处理？

<?php
namespace frontend\controllers;
use yii\web\Controller;
use yii\db\Migration;
use yii\db\Schema;
class TableController extends Controller
{
    public function actionIndex()
    {
        $mir = new Migration();  
        $sch = new \yii\db\mysql\Schema;  
        $tableName='shang';

        $table=$db->createCommand("SHOW TABLES LIKE '".$tableName."'")->queryAll();
        if($table==null)        {
            echo '1';
        }else{
            echo '2';
        }

        // $mir->createTable('shang', [  
        //     'id' => 'pk',  
        //     'title' => $sch::TYPE_STRING . ' NOT NULL',  
        //     'content' => $sch::TYPE_TEXT,  
        // ]);  

    }

}