如何在 PHP 中使用 While 循环？

Question

我在最简单的代码上遇到了麻烦。由于某种原因，以下代码仅检索数据库中的第一行。我正在尝试 while 循环，但它对我不起作用。这是我的代码：

<?php


//Connect to the database
require_once('mysql_connect.php') ;

$query = "SELECT * FROM past_due_students WHERE charged_today = 'No' ORDER BY past_due_id" ;
$result = mysqli_query($dbc, $query) ;
$number_of_students = mysqli_num_rows($result) 

if ($number_of_students >= 1) {

    //Loop through the entire table
    while ($row = mysqli_fetch_array($result, MYSQLI_BOTH)) {

        $student_id = $row['student_id'] ;
        $number_of_declines = $row['number_of_declines'] - 1;

        //Update the number of declines
        $query = "UPDATE past_due_students SET number_of_declines = $number_of_declines WHERE student_id = $student_id" ;
        $result = mysqli_query($dbc, $query) ;
        $number = mysqli_affected_rows($dbc) ;

        if ($number == 1) {

            echo '<p><b>The number of declines has been successfully updated.</b></p>' ;

        } else {
            echo $query ;
        }

    }//END while loop

}//END if ($number_of_students >= 1) {

?>

它只抓取第一行，而不抓取其他行。

Answer 1

您正在覆盖 $result 的值：

$query = "SELECT * FROM past_due_students....." ;
$result = mysqli_query($dbc, $query) ;

//Loop through the entire table
 while ($row = mysqli_fetch_array($result, MYSQLI_BOTH)) {

    $query = "UPDATE past_due_students SET  ...." ;
    $result = mysqli_query($dbc, $query) ; <-- OVERWRITTING HERE.

使用不同的变量来保存内部查询结果对象。

Answer 2

通过将下一个查询资源分配给同一变量，您将破坏从 SELECT 查询获得的 $result 资源。

请为 UPDATE 查询使用不同的变量。

因此，将其更改

$query = "UPDATE past_due_students SET number_of_declines = $number_of_declines WHERE student_id = $student_id" ;
$result = mysqli_query($dbc, $query) ;

为：

$query = "UPDATE past_due_students SET number_of_declines = $number_of_declines WHERE student_id = $student_id" ;
$updateResult = mysqli_query($dbc, $query) ;

Answer 3

尝试使用其他变量名称更改更新查询的 $result

Answer 4

您正在覆盖循环中的 $result var，它看起来像。此外，您的第二个 mysqli_affected_rows 正在传递数据库对象。

更改

$result = mysqli_query($dbc, $query) ;
$number = mysqli_affected_rows($dbc) ;

为

$result2 = mysqli_query($dbc, $query) ;
$number = mysqli_affected_rows($result2) ;