Wordpress get next, preivous attachment

gistfile1.aw

function rb_get_next_attachment( $post_id, $previous = false ){
	global $wpdb;
	$post = get_post($post_id);

	$current_post_date = $post->post_date;
	if( $previous ){
		$order = 'DESC';
		$where = "p.post_date < '$current_post_date'";
	}else{
		$order = 'ASC';
		$where = "p.post_date > '$current_post_date'";
	}
	$query = "SELECT p.* FROM wp_posts AS p  
				INNER JOIN wp_term_relationships AS tr 
					ON p.ID = tr.object_id 
				INNER JOIN wp_term_taxonomy tt 
					ON tr.term_taxonomy_id = tt.term_taxonomy_id 
					AND tt.taxonomy = 'category'  
				WHERE $where 
					AND p.post_type = 'attachment' 
					AND p.post_status = 'inherit' 
					AND tt.taxonomy = 'category' 
				ORDER BY p.post_date $order LIMIT 1";
	$result = $wpdb->get_row($query);
	if ( null === $result )
		$result = '';

	return $result;
}