控制產品庫存量的php語法

php語法：
<?php
   $colname_web_shop2017 = "-1";
if (isset($_GET['p_id'])) {
  $colname_web_shop2017 = $_GET['p_id'];
}
mysql_select_db($database_conn_web, $conn_web);
$query_web_shop2017 = sprintf("SELECT shop2.p_id, shop2.p_name, shop2.p_price, shop2.p_open, shop2.p_pic, shop2.inventory,  orderlist.p_name,orderlist.odlist_ok, orderlist.odlist_qty, SUM(  `orderlist`.`odlist_qty` ) AS orderlistNum ,shop2.inventory AS inventory , orderlist.odlist_ok AS odlist_ok ,orderlist.odlist_qty AS odlist_qty
FROM shop2
LEFT JOIN orderlist ON shop2.p_name = orderlist.p_name  and  orderlist.odlist_ok='y'
GROUP BY  `orderlist`.`odlist_ok` ,  `shop2`.`p_id` , `shop2`.`p_name` ,  `orderlist`.`p_name`
HAVING  p_id = %s ", GetSQLValueString($colname_web_shop2017, "int"));
$web_shop2017 = mysql_query($query_web_shop2017, $conn_web) or die(mysql_error());
$row_web_shop2017 = mysql_fetch_assoc($web_shop2017);
$totalRows_web_shop2017 = mysql_num_rows($web_shop2017);//段落

?>

<?php
$a=$row_web_shop2017['inventory'];
$b=$row_web_shop2017['orderlistNum'];
$c=$row_web_shop2017['op_name'];
$d=$row_web_shop2017['odlist_ok'];

?>



內文：
 <?php if ($b == NULL    ){ ?>
 已購數量 <?php echo 0 ?>
 <?php } // Show if recordset not empty ?>

  <?php if ($b !== NULL   ){ ?>
  已購數量 <?php echo $b ?>
 <?php } // Show if recordset not empty ?>


<?php if ($b == NULL  ){ ?>
  &nbsp;&nbsp;&nbsp;庫存 <?php echo $a; ?>

 <?php } // Show if recordset not empty ?>

 <?php if ($b !== NULL  and $a-$b>=0){ ?>
  &nbsp;&nbsp;&nbsp;庫存 <?php echo $a-$b; ?>

  <?php } elseif ($b !== NULL  and $a-$b<0){ ?>
   &nbsp;&nbsp;&nbsp;庫存 <?php echo 0 ?>
 <?php } // Show if recordset not empty ?>

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