MySQL DB更新2值：1增加1减少

Question

I have a table in my database called member within this table are the fields 'personID&balance`.

If a user 1 has a balance of 20.00 and wishes to send 10.00 to user 2 what is the best way to reduce user 1's balance and update user 2's balance?

Any help/advice would be great!

Thanks in advance

UPDATE

CODE:

 $accountfrom=$_POST[accountfrom];
 $accountto=$_POST[accountto];
 $amount=$_POST[amount]; 
$con = mysql_connect("localhost","cl49-XXX","XXX");
if (!$con) 
  {
   die('Could not connect: ' . mysql_error());
   }

mysql_select_db("cl49-XXX", $con)or die( "Unable to select database");


$result = mysql_query("UPDATE member
SET balance = IF(personID = $acountfrom, balance-$amount, balance+$amount)
WHERE personID IN ($accountfrom, $accountto)")
 or die(mysql_error());

RESULT:

'You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' balance-100.00, balance+100.00) WHERE personID IN (84587745 , 98554130)' at line 2'

Answer 1

UPDATE member
SET balance = IF(personID = 1, balance-10, balance+10)
WHERE personID IN (1, 2)

To limit the transfer to the sender's balance:

UPDATE member m1
CROSS JOIN (SELECT LEAST(10, balance) transfer
      FROM member
      WHERE personID = 1) m2
SET m1.balance = IF(personID = 1, balance - transfer, balance + transfer)
WHERE personID in (1, 2)