我需要检索图库中图像的URL，基于帖子ID，适合的SQL查询是什么？

我目前在via上找到了如下内容google, 但我不知道如何从外部Wordpress访问全局WP变量-我需要做什么？

function getGallery($id)
{
 global $wpdb;

 //SQL query to retrieve all attachment of mime type image/jpeg from the given post
 $querystr = "
     SELECT ID, post_name, guid, meta_value
     FROM $wpdb->posts wposts
     INNER JOIN $wpdb->postmeta meta ON meta.post_id = wposts.ID AND meta.meta_key = \'_wp_attachment_metadata\'
     WHERE wposts.post_type = \'attachment\'
     AND wposts.post_parent = \'".$id."\'
     AND wposts.post_mime_type = \'image/jpeg\' 
 ";

 //get result set from the query
 $pictures = $wpdb->get_results($querystr, ARRAY_A);

 //return resultset
 return $pictures;
}