在IF语句中更新数据库

Question

I am trying to update my database inside an IF statement but it doesn't seem to be working. email_sent doesn't change to 1. Is my statement correct?

$result2 = mysql_query ("SELECT * FROM stock_control WHERE id = '$id' ");

while ($row1 = mysql_fetch_array($result2))
{
$item=$row1['item'];
$location=$row1['location'];
$quantity=$row1['quantity'];
$threshold=$row1['threshold'];
$emailSent=$row1['email_sent'];
}


if ($quantity <= $threshold && $emailSent == 0) {
mysql_query("UPDATE stock_control SET quantity=quantity - '$n_quantity', 
email_sent = '1' WHERE id = '$id' ");
} else {
mysql_query("UPDATE stock_control SET quantity=quantity - '$n_quantity' WHERE 
id = '$id' ");
}

Answer 1

You are closing your loop too fast for the while. You are just getting the last value of the loop:

$result2 = mysql_query ("SELECT * FROM stock_control WHERE id = '$id' ");

while ($row1 = mysql_fetch_array($result2))
{
   $item=$row1['item'];
   $location=$row1['location'];
   $quantity=$row1['quantity'];
   $threshold=$row1['threshold'];
   $emailSent=$row1['email_sent'];

   if ($quantity <= $threshold && $emailSent == 0) {
      mysql_query("UPDATE stock_control SET quantity=quantity - '$n_quantity', 
      email_sent = '1' WHERE id = '$id' ");
   } else {
      mysql_query("UPDATE stock_control SET quantity=quantity - '$n_quantity' WHERE 
      id = '$id' ");
   }
}

Answer 2

Check on the $n_quantity variable, it does not seem to be properly defined. Did you mean $quantity instead?

You may not need the while loop, since you are dealing with only one row of the table, the row with the given id. Otherwise, if you had more rows, the while loop as given would have been prematurely closed.

SELECT * could select too many columns, which might be disadvantageous. Also, the use of msyql_query is deprecated, you will need to use mysqli_query or PDO. So the following might be helpful, assuming that $con is your database connection:

$result2 = mysqli_query($con,"SELECT item, location, quantity, threshold, email_sent from stock_control where id = '$id'");

list($item,$location,$quantity,$threshold,$emailSent) = mysqli_fetch_array($result2);

if ($quantity <= $threshold && $emailSent == 0) {
      mysqli_query($con,"UPDATE stock_control SET quantity=quantity - '$quantity', 
      email_sent = '1' WHERE id = '$id' ");
   } else {
      mysqli_query($con,"UPDATE stock_control SET quantity=quantity - '$quantity' WHERE id = '$id' ");
   }

Answer 3

The resolution was time related to the query and therefore I needed to close the connection and open a new connection after my HTML to query the DB.

Many thanks to all for your comments as they were all very useful and will help me in the future.

:)