查询限制越来越忽略

Question

我有这样的代码及其计算所有列correct行和igonring代码LIMIT 5

行多数民众赞成越来越忽略

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

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这里是整个代码

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<?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; 

?> 

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我曾尝试以下方法

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select sum(correct) from exams where username 
= :username ORDER BY testID DESC LIMIT 0,5 

和

select sum(correct) from exams where username 
    = :username ORDER BY testID DESC LIMIT 5 

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Answer 1

LIMIT限制结果的数量则返回

您的SUM总是返回1个结果，所以您的限制将不会执行任何操作。

如果需要，您可能需要像这样（未经测试，只是举例）

SELECT sum(correct) FROM(
    select correct from exams where username 
    = :username DESC LIMIT 5 
) 

我删除了顺序，摆弄它。