PHP + MySQL - While 循环不是

Question

我对 PHP + MySQL 还很陌生，所以请原谅我的困惑—— 我不明白为什么我的代码不会遍历 'underground_name' 列并回显所有行？

这是我的代码...

<?php
    include("../includes/header.php");
    require('../../mysqli_connect.php');
    include("../functions/filter_time.php");

    // query the database
    $query = "SELECT * FROM projects_underground, underground, projects
            LEFT JOIN river ON projects.river_id=river.river_id 
            LEFT JOIN dlr ON projects.dlr_id=dlr.dlr_id 
            LEFT JOIN overground ON projects.overground_id=overground.overground_id 
            LEFT JOIN natrail ON projects.natrail_id=natrail.natrail_id 
            LEFT JOIN tram ON projects.tram_id=tram.tram_id

            WHERE 
            projects_underground.underground_fk = underground.underground_id AND
            projects_underground.projects_fk = projects.projects_id AND
            name = 'Imperial War Museum'";

    $result = mysqli_query($dbc, $query); // put queried result into variable
    $row = mysqli_fetch_assoc($result); // put that into seperate arrays called $row
    $ug = mysqli_fetch_array($result);

    while ($ug = mysqli_fetch_array($result)) {
        echo $ug['underground_name'];
    }
?>

提前感谢您的帮助！

Answer 1

1. 尝试在 SQL 控制台上运行您的查询，并检查您得到的结果。
2. 尝试转储$ug 数组以查看是否正在获取输出。
3. 您调用了三次mysqli_fetch_*()，一次是在初始化$row 时，一次是在初始化$ug 时，一次是在while 循环中。这将错过前两行 - 因此，如果查询结果中只有两行，则不会打印任何输出。

$row = mysqli_fetch_assoc($result); // Fetches first result row off the stack 
$ug = mysqli_fetch_array($result);  // Fetches second result row

while ($ug = mysqli_fetch_array($result)) { // Fetches third row
     // Starts printing from the third row (if there is any)
     echo $ug['underground_name'];
}

Answer 2

您需要了解，当您调用 mysqli_fetch_assoc($result); 或 mysqli_fetch_array($result); 时，您正在从结果堆栈中获取一行。因此，在您到达while 语句之前，您已经完成了两行。因此，如果您的查询返回三个结果，那么您只会在循环中显示一个。

Answer 3

尝试运行以下命令查看返回的行数：

echo $result->num_rows().'行已返回。'.PHP_EOL;

这是我认为应该可以解决问题的代码更新（通过删除早期对 fetch_assoc 和 fetch_array 的调用并添加一些基本检查是否已返回任何行）：

<?php
include("../includes/header.php");
require('../../mysqli_connect.php');
include("../functions/filter_time.php");

// query the database
$query = "SELECT * FROM projects_underground, underground, projects
        LEFT JOIN river ON projects.river_id=river.river_id 
        LEFT JOIN dlr ON projects.dlr_id=dlr.dlr_id 
        LEFT JOIN overground ON projects.overground_id=overground.overground_id 
        LEFT JOIN natrail ON projects.natrail_id=natrail.natrail_id 
        LEFT JOIN tram ON projects.tram_id=tram.tram_id

        WHERE 
        projects_underground.underground_fk = underground.underground_id AND
        projects_underground.projects_fk = projects.projects_id AND
        name = 'Imperial War Museum'";

$result = mysqli_query($dbc, $query); // put queried result into variable

if (is_object($result) && $result->num_rows() > 0) {

    // This line is just for testing, delete from real code
    echo $result->num_rows().' rows have been returned.'.PHP_EOL;

    while ($ug = mysqli_fetch_array($result)) {
        echo $ug['underground_name'];
    }
} else {
    // Do something if no results were returned
}
?>