Custom $wpdb queries.. need help

I’m sure this looks horrible, but it is my first time doing a custom query using $wpdb

I am trying to grab the total values from a meta_value in the commentmeta table assigned a specific post id with a specific meta_key of ‘rating’.. but I’m not sure what I’m doing wrong.. here is my code

<?php
			global $wpdb;
   			$rating = $wpdb->get_var("
   				SELECT SUM($wpdb->commentmeta.meta_value)
   				FROM $wpdb->posts, $wpdb->commentmeta, $wpdb->comments
   				WHERE $wpdb->posts.ID='46'
   				AND $wpdb->commentmeta.meta_key='rating'");

   				echo $rating;
			?>