PHP | left join avg query give me Array()

You have a mistake in your query

      $sql = "SELECT 
                    {$wpdb->prefix}commentmeta.comment_id, avg({$wpdb->prefix}commentmeta.meta_value) avg_meta_value
                FROM 
                    {$wpdb->prefix}commentmeta
                LEFT JOIN 
                    {$wpdb->prefix}comments
                ON 
                    {$wpdb->prefix}commentmeta.comment_id = {$wpdb->prefix}comments.comment_ID
                WHERE 
                    {$wpdb->prefix}commentmeta.meta_key = 'rating'
                GROUP BY 
                    {$wpdb->prefix}commentmeta.comment_id 
                ORDER BY 
                    avg_meta_value desc";
                    
        $results = $wpdb->get_results($sql);
        
        foreach ($results as $result)
        {
            echo $result->comment_id.'<br>';
        }