WHERE MySQL clause not working in Drupal 6
Try

$query3 = db_query("SELECT nid FROM {node} WHERE uid = '%d'", 19);


this is the proper way to handle variables in the db_query, as it prevents sql injection.

//grab the current user
global $user;
$query3 = db_query("SELECT nid FROM {node} WHERE uid = '%d'", $user-&gt;uid);