PDO没有调用行的顺序

Question

我试图使这项工作，但它加起来所有的行，而不是像我在查询中尝试的那样

$username = $_SESSION['username'];
$stmt = $db->prepare("
select sum(correct) from exams where 
username = :username ORDER BY :testID DESC LIMIT :5");

由于无法调用最后5行的问题，我无法测试排序字符串。

我已经在stackoverflow上阅读了与此类似的问题，但似乎仍然无法做到正确。 也许我的代码还有其他问题但我总是可以添加，如果需要:)

完整的代码

   <?php

require('includes/config.php'); 

//if not logged in redirect to login page
if(!$user->is_logged_in()){ header('Location: login.php'); } 
$username = $_SESSION['username'];
$last5rate = $db->prepare("select sum(correct) from exams where username = :username ORDER BY :testID DESC LIMIT 5");
$last5rate->execute(array(':username' => $username));
for($i=0; $rows = $last5rate->fetch(); $i++){
    //Edit this row
$last5  = $rows['sum(correct)'];
$last5final = $last5 / 10;
 }
echo $last5final;

?>

Answer 1

如果您没有约束限制，则更改您的查询

select sum(correct) from exams where username = :username ORDER BY :testID DESC LIMIT :5
$last5rate->execute(array(':username' => "$username"));

for($i=0; $rows = $last5rate->fetch(); $i++){
    $last5  = $rows['sum(correct)'];
 }

至

select sum(correct) as correct from exams where username = :username ORDER BY :testID DESC LIMIT 5
$last5rate->execute(array(':username' => $username,':testID' => $testID));
while ($row = $last5rate->fetch(PDO::FETCH_ASSOC)) {
   echo $row['correct']; // or use it as you want
}