mysql不会在while语句中更新

Question

I am trying to extract two fields from tables accesscode and student the update the table borrowers with the data that i have extracted from the previous tables.

$q=$db->query("SELECT regnum,  accesscode FROM student,student_accesscode WHERE student.id=student_accesscode.studentid");


while($qd=$q->fetch(PDO::FETCH_ASSOC))
{
$access=$qd['accesscode'];
$regnum=$qd['regnum'];  
$q2=$db->exec("UPDATE borrowers SET cardnumber='$access' WHERE cardnumber='$regnum'");
if($q2)
{
echo $access.'          '. $regnum.'<br/>';     
}
else
{
echo'erro....<br/>';
}
}
?>

Answer 1

Appending $db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING); before $db->query() probably gives you an Exception...

Prior to the $db->setAttribute , you will get this:

PDO::prepare(): SQLSTATE[HY000]: General error: 2014 Cannot execute queries while other unbuffered queries are active. Consider using PDOStatement::fetchAll(). Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO::MYSQL_ATTR_USE_BUFFERED_QUERY attribute.

So, instead of fetch() , use fetchAll() with foreach loop, it will make you less insane.

Ref. from php.net

Answer 2

For a statement that you need to issue multiple times, prepare a PDOStatement 
object with PDO::prepare() and issue the statement with PDOStatement::execute(). 

From the PHP exec page at http://www.php.net/manual/en/pdo.exec.php

I don't know if this is the problem, I always use prepare and execute . It could just be for performance reasons. Something to try anyway.