something like this:
<?php
$rsSubCategories = new WA_MySQLi_RS("rsSubCategories",$DatabaseConn,0);
$rsSubCategories >setQuery("SELECT * FROM sub_categories WHERE SubCategoryID = ? ORDER BY SubCategoryName ASC");
$rsSubCategories->bindParam("i", "".($rsCategories->getColumnVal("CategoryID")) ."", "-1"); //paramCategoryID
$rsSubCategories >execute();
?>