how to update by IdItem - php

Question

To master2 in php, mysql. I'm new in php about 2 month
I need help. about update table.
I have page that display current table. when I click update button update, I want row 1, quantity in current table will decrease by IdItem from result table. So the equation will like this in current table quantity=quantity-quantityPass. So only field Item change quantity result. Now when I click update, it effect to all in table current result. any solution?
table for 'result'
+--------+-------------+--------------+-----------+--------+--------------+
| IdItem | username | rentItem | quantity | result | quantityPass |
+--------+-------------+--------------+-----------+--------+--------------+
| 84 | FahmiNazirul| Speaker | 1 | PASS | 1 |
+--------+-------------+--------------+-----------+--------+--------------+
| 86 | Andy | Keyboard | 3 | PASS | 2 |
+--------+-------------+--------------+-----------+--------+--------------+
| 89 | FahmiNazirul| Speaker | 5 | PASS | 3 |
+--------+-------------+--------------+-----------+--------+--------------+
table for current quantity 'update'
+--------+-------------+--------------+
| Id | Item | quantity |
+--------+-------------+--------------+
| 1 | Speaker | 10 |
+--------+-------------+--------------+
| 2 | Keyboard | 10 |
+--------+-------------+--------------+
tableupdate.php
$result = mysql_query("SELECT * FROM update where Item = Item");
echo "<table border='1'>
<tr>
<th>Item</th>
<th>Quantiti</th>
<th></th>
</tr>";
while($row = mysql_fetch_array($result))
{
$item= $row['Item'];
echo " <tr> ";
echo " <td> " . $row['Item'] . " </td> ";
echo " <td> " . $row['quantity'] . " </td> ";
echo " </tr> ";
}
echo "</table>";
echo " <a href ='quantityupdate.php?Item=$item' onclick='return Confirm_Box()' >Update</a>";
quantityupdate.php
$item =$_REQUEST['Item'];
// sending query
mysql_query("UPDATE update ,result SET update.quantity=update.quantity-result.quantityPass where update.Item = result.rentItem")
or die(mysql_error());
header("Location: update.php");

Answer 1

Use UPDATE with JOIN like this:
UPDATE kemaskini AS u
INNER JOIN result AS r ON u.Item = r.rentItem
SET u.quantity = u.quantity - r.quantityPass;
For the duplicate entries of the Item in the first table result you can do this:
UPDATE kemaskini AS u
INNER JOIN
(
SELECT rentItem, SUM(quantityPass) AS TotalquantityPass
FROM result
GROUP BY rentItem
) AS r ON u.Item = r.rentItem
SET u.quantity = u.quantity - r.TotalquantityPass;
SQL Fiddle Demo

Answer 2

mysql_query("UPDATE update ,result SET update.quantity=update.quantity-result.quantityPass where result.rentItem='$item' AND update.Item='$item'")