php - 查询限制被忽略

Question

我有这段代码，它计算列中的所有行并correct计算LIMIT 5

被忽略的代码行

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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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我尝试了以下方法

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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

---

Answer 1

LIMIT限制它返回的结果数量

你SUM总是返回 1 个结果，所以你的限制不会做任何事情。

您可能需要这样的东西（例如，未经测试）

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

我删除了订单，根据需要摆弄它。