wpdb to get user id -> problem

Hi all,

I’m currently trying to write a plugin and i’m getting a little bug i can’t understand.

I’m using wpdb to retrieve the author id (i’m running on wpmu but i think that that’s not a problem no?).

This is my function:

function getLastArticle($author){
		//echo $author;
		global $wpdb, $post;
		$pre = $wpdb->prefix;
		/*global $domain;
		$author_id = $wpdb->get_var("SELECT user_id FROM wp_usermeta WHERE user_login = {$author} LIMIT 1");
		$user_info = get_userdata($user_id);
		//echo "Login: ".$user_info->user_login;
		*/

		/*$query = "SELECT ID from $wpdb->users WHERE user_login={$author}";
		$author_id = $wpdb->get_var($query);
		$wpdb->print_error();*/

		$last_posts = (array)$wpdb->get_results("
			SELECT ID, post_title, post_date
			FROM wp_posts, wp_term_relationships, wp_term_taxonomy
			WHERE post_author = 0
			AND {$pre}term_relationships.object_id = {$pre}posts.ID
			AND {$pre}term_relationships.term_taxonomy_id = {$pre}term_taxonomy.term_taxonomy_id
			AND post_status = 'publish'
			AND post_date < NOW()
			AND post_type = 'post'
			GROUP BY ID
			ORDER BY post_date
			LIMIT 5
		");
$liste = "<ul class=\"iapListPosts\"> |$author";

		foreach($last_posts as $thepost){
			$liste .= "<li><a href=\"".get_permalink($thepost->ID)."\">".$thepost->post_title."</a></li>";
		}
		$liste .= "$author_id</ul>";
		return $liste;

But it returns me absolutly nothing in the foreach, even the $author_id is null.
I’m using a shortcode to display all the results.

Any idea plase ?