WooCommerce使用wpdb查询获取customer_user_agent

Question

如何使用$ wpdb查询从数据库中获得_customer_user_agent？

我尝试了这个，但是没有用：

   global $wpdb;
   $query = "
   SELECT DISTINCT SUM(pm.meta_value)
   FROM {$wpdb->prefix}posts as p
   INNER JOIN {$wpdb->prefix}postmeta as pm ON p.ID = pm.post_id
   WHERE p.post_type LIKE 'shop_order'
   AND pm.meta_key LIKE '_customer_user_agent'
   ";

   $dev_results = $wpdb->get_var($device_query);
   $device_type = $dev_results;

我希望结果像这样： ipad-iphone-macintosh ...