how to SELECT meta values that are not null?

I founded myself my searching:

$sql = $wpdb->prepare( "
    SELECT meta_value 
    FROM {$wpdb->prefix}commentmeta 
    INNER JOIN {$wpdb->prefix}comments ON {$wpdb->prefix}commentmeta.comment_id = {$wpdb->prefix}comments.comment_ID 
    WHERE comment_post_ID = %d AND meta_key = 'rating' AND meta_value IS NOT NULL AND meta_value <> '' ", get_the_ID() 
);
$results = $wpdb->get_results( $sql );