Warning:mysql_fetch_array() expects parameter 1 to be resour

Warning:mysql_fetch_array() expects parameter 1 to be resource boolean given in
第一次运行会出现这个,但是刷新之后就没事儿了,更奇怪的是之后多次运行都没事儿,忽然某一次运行又出现这个错误提示.再次刷新,又没事儿了.这是为什么?
代码如下
require("connect.php"); //Connecting to database.
$db_name = "PatientDatabase";
mysql_select_db($db_name); //Select patient database.
//Query to check patient enrollment timestamp.
$query = "SELECT * FROM patienttable";
$result = mysql_query($query) or die(mysql_error());
while($row = mysql_fetch_array($result)){
$date_time = strtotime($row['Timestamp']);
$phone_number = $row['PhoneNumber'];
$first_name = $row['FirstName'];
$last_name = $row['LastName'];
$days = $row['Days'];
$expire_day = $row['ExpireDay'];
$status = $row['Status'];
$pid = $row['user_ID'];
.......................................
}
每次错误都是提示While行出现问题