php - 为什么 PHP 中的 mySQL 返回空结果？

Question

我正在尝试将 ~55K 记录从 mySQL 服务器迁移到 MongoDB。我无法通过任何易于访问的方法(例如 JSON/CSV 导入)来执行此操作，因为数据存储方法(其结构方式)将非常不同。因此，我在 php 中创建了一个脚本来执行此操作。

我遇到的问题是，在大型数据集上(即使较小的数据集包含问题条目，也无法使用较小的数据集重现)，尽管条目存在，但查询偶尔会报告没有数据。它绝对存在，因为当 php 直接访问该特定条目或它包含在较小的数据集中时，它工作得很好。例如，在导入文本文件时，我只收到~42k/54k 记录。

在我通过调用 php 文件的 url 收到的回显中，我显示该查询被调用了正确的次数，但有许多记录显示为不存在，因此应答回显是空白的。代码如下:

//Makes a connection to the database
$conn = makeConnection();

$filename = '/home/dbserverdownload.txt';
$file = fopen($filename, 'a');

$sql = "SELECT * FROM maintable ORDER BY ID DESC LIMIT 1";
$resultID = mysqli_query($conn, $sql);
$ID = mysqli_fetch_object($resultID);
echo $ID->ID;
//loops through the database and appends the data to the file as it goes
for($var=2; $var <= $ID->ID; $var++){
    $sql1 = "SELECT * FROM servertable WHERE ID = '$var'";
    $result1 = mysqli_query($conn, $sql1);
    $values = mysqli_fetch_object($result1);

    $id =  $values->ID;
    $ip = $values->IP;
    $port = $values->port;
    $running = $values->running;
    $afk = $values->afk;
    $gamemode = $values->gamemode;
    $maxplayers = $values->maxplayers;
    $spawnprotection = $values->spawnprotection;
    $whitelist = $values->whitelist;
    $enablequery = $values->enablequery;
    $enablercon = $values->enablercon;
    $rconpassword = $values->rconpassword;
    $motd = $values->motd;
    $achievements = $values->announceplayerachievements;
    $allowflight = $values->allowflight;
    $spawnanimals = $values->spawnanimals;
    $spawnmobs = $values->spawnmobs;
    $forcegamemode = $values->forcegamemode;
    $hardcore = $values->hardcore;
    $pvp = $values->pvp;
    $difficulty = $values->difficulty;
    $generatorsettings = $values->generatorsettings;
    $levelname = $values->levelname;
    $levelseed = $values->levelseed;
    $leveltype = $values->leveltype;
    $autosave = $values->autosave;

if($ip == "148.57.44.10"){
  //if the server is server1
  $servername = "server1".$port;
} else if ($ip == "165.108.22.199"){
  //if the server is server2
  $servername = "server2".$port;
} else{
  $servername = "";
}



//Adds all content that was already gained to the JSON string
$startingContent = "{\"_id\":\"$servername\",
                    \"ip\":\"$ip\",
                    \"port\":\"$port\",
                    \"running\":\"$running\",
                    \"afk\":\"$afk\",
                    \"gamemode\":\"$gamemode\",
                    \"maxplayers\":\"$maxplayers\",
                    \"spawnprotection\":\"$spawnprotection\",
                    \"whitelist\":\"$whitelist\",
                    \"enablequery\":\"$enablequery\",
                    \"enablercon\":\"$enablercon\",
                    \"rconpassword\":\"$rconpassword\",
                    \"motd\":\"$motd\",
                    \"announceplayerachievements\":\"$achievements\",
                    \"allowflight\":\"$allowflight\",
                    \"spawnanimals\":\"$spawnanimals\",
                    \"spawnmobs\":\"$spawnmobs\",
                    \"forcegamemode\":\"$forcegamemode\",
                    \"hardcore\":\"$hardcore\",
                    \"pvp\":\"$pvp\",
                    \"difficulty\":\"$difficulty\",
                    \"generatorsettings\":\"$generatorsettings\",
                    \"levelname\":\"$levelname\",
                    \"levelseed\":\"$levelseed\",
                    \"leveltype\":\"$leveltype\",
                    \"autosave\":\"$autosave\"
                  }";



echo $startingContent."<br/>";


//This is the JSON data that will be passed to mongo

if(strlen($ip)>6){
    if (fwrite($file, $startingContent) === FALSE) {
        echo "Cannot write to file ($filename) with $startingContent";
        exit;
    }
}
}

我还尝试过一个查询，该查询将大量(全部、一半、四分之一等)结果提取到一个 block 中，而不是大量的单独查询。该实验的最终结果是，每次运行时都会更新可变数量的记录(通常是 400 到 4000 之间看似随机的数字)。有谁知道为什么会发生这种情况？如果没有，我是否应该编写自己的程序来迭代可以从 mySQL 导出的 CSV？

Answer 1

首先，替换您的查询$sql = "SELECT * FROM maintable ORDER BY ID DESC LIMIT 1"; 使用此 $sql = "SELECT MAX(ID) as ID FROM maintable"; 以获得更好的性能。

这里的问题是，您可能有一个返回 false 的查询，然后您的应用程序就会终止。例如，假设您的循环尝试查询 ID=3，但数据库中没有具有此 ID 的行。然后下一行抛出一个您没有看到的异常。您可以在您的开发机器 php.ini 中使用 set display_errors=On。

使用它来检查您是否有要获取的结果:

....
for($var=2; $var <= $ID->ID; $var++){
    $sql1 = "SELECT * FROM servertable WHERE ID = '$var'";
    $result1 = mysqli_query($conn, $sql1);

    if($result) {
        $values = mysqli_fetch_object($result1);

        $id =  $values->ID;
        $ip = $values->IP;
        ....