我刚刚发出警告。我想知道这个警告意味着什么，我怎么能摆脱它呢？
Warning: mysql_result(): Unable to jump to row 0 on MySQL result index 10 in /public_html/domain/online.php on line 56
基本上是我第二次从用户那里选择时间。这是代码
1st selct - nothing happends - selecting from user_id
2nd select - selecting from online
$sql = "SELECT time FROM user WHERE update = 'Online'";
$result = mysql_query($sql);
$time = mysql_result($result, 0);
$update = time() - $time;
第二次在选择用户时间时运行结果时崩溃了。那个php警告是什么意思，我该如何修复它？