php - PDO 不调用一行的顺序

Question

我已经尝试完成这项工作，但它会将所有行加起来，而不仅仅是像我在查询中尝试的那样 5

$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。

我在 stackoverflow 上阅读过类似的问题，但似乎仍然无法正确回答。也许我的代码还有其他问题，但如果需要，我可以随时补充 :)

完整代码

   <?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

如果你没有绑定(bind)限制，那么改变你的查询

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)'];
 }

到

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
}