problem with SQL execution in shopping cart

hi everyone,

i have a shopping cart with those SQL queries: add product to cart, update product (product quantity) and delete product from cart. when i execute the add, update or delete SQL query the SELECT QUERY it doesn't work. These are my queries:

<?php if ($cart == "add") {
$sql="INSERT INTO cart (sessionid, code, model,descr,pricer,qty) VALUES ('".$sessionid."','".$code."','".$model."','".$descr."','".$pricer."','".$qty."')";

if (!mysql_query($sql,$con))
  {
  die('Error: ' . mysql_error());
  }
mysql_close($con) 
?><?php } else { ?><?php } ?>

<?php if ($cart == "del") {
mysql_query("DELETE FROM cart WHERE code='".$code."'");

mysql_close($con);
?>deleted ok<?php } else { ?><?php } ?>

<?php if ($cart == "upd") {
mysql_query("UPDATE cart SET qty='".$qty."' WHERE code='".$code."'");

mysql_close($con);
?>updated ok<?php } else { ?><?php } ?>

<?php
$result = mysql_query("SELECT * FROM cart WHERE sessionid='".$sessionid."' ORDER BY pricer");
?>

PHP:

another problem. how i can display a message (This product is already in the cart) when the visitor adds a product that is already in the cart?

thanks in advance.