Custom Query by author and post ID

Hi,
I need to display a post that has a specific ID (obtained from the custom field of the current post) and (for extra safety reasons) the author ID is the same as the current post.

I think I have to use a custom query, something like this:

<?php
$thisuser_id = get_the_author_ID();
$main_related_post = get_post_meta($post->ID, main_post, true);

global $wpdb;
global $post;
$querystr = "
SELECT wposts.*
FROM $wpdb->posts wposts
		AND wposts.post_status = 'publish'
		AND ID = '$main_related_post'
		AND AUTHOR ID = '$thisuser_id'
ORDER BY wpostmeta.meta_value ASC
LIMIT 1
";
$pageposts = $wpdb->get_results($querystr, OBJECT);
if ($pageposts):?>
   <small>
   <?php _e("Main Related Post:"); ?>
   </small>
		<ul>
            <li class="article">
					<a class="artilink" href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title_attribute(); ?>">
						<?php the_title(); ?>
					</a>
            </li>
		</ul>
<?php endif; ?>

please forgive my bad fancy code..
thanks for your attention