multiple tables select exclude

[phpjaco]

I have successfully created a script that filters through all the fields in the one to many relational table (see below) - and prints them out

$sql = 'SELECT articleID,relatedID FROM article_to_related WHERE articleID ='.$_GET['id'];
			$rs = @mysql_query($sql,$conn);
			if ($rs) {
			   $listAllArticle = '<select name="allArticles[]" multiple class="bodycopylinks" style="width:300px;" size="4">';
			   while ($row2 = @mysql_fetch_array($rs)) {
			      // NOW SELECT ARTICLE ID, HEADING
				  $sql3 = 'SELECT sub_article_id,heading FROM sub_article WHERE sub_article_id='.$row2['relatedID'];
				  //echo $sql3;
				  $rs3 = @mysql_query($sql3,$conn);
				  if ($rs2) {
				  while ($row3 = @mysql_fetch_array($rs3)) {
			      $listAllArticle .= '<option value="'.$row3['sub_article_id'].'">'.$row3['heading'].'</option>';
				  }
				  }
			   }
			   $listAllArticle .= '</select>';
			}		
			echo $listAllArticle;  

this prints out the information on the articles that are in the table, how do i do the opposite in other words, print out all the articles that are not inside the table?

thanks in advance.