PDOStatement 包含什么以及 fetch/fetchAll 的用途是什么? [英] What does a PDOStatement contain and what is the use of fetch/fetchAll?

问题描述

我无法理解 1) PDOStatement 对象内部是什么以及 2) 为什么我需要 fetch() 或 fetchAll() 方法.

I cannot understand 1) what is inside a PDOStatement object and 2) why I would need a fetch() or fetchAll() method.

我的数据库:一个名为animals"的简单表，包含 3 列(id、名称、物种).

My database: One simple table called 'animals' with 3 columns (id, name, species).

我的代码:

try {
     $pdo = new PDO($dsn, $user, $pass);
     $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
} catch (PDOException $e) {
     exit( $e->getMessage() );
}

try {
     $sql = "SELECT * FROM animals";
     $results = $pdo->query($sql);
     print_r($results);
} catch (PDOException $e) {
     echo $e->getMessage();
}

首先，print_r(或var_dump)打印:PDOStatement Object ( [queryString] => SELECT * FROM animal )

First off, print_r (or var_dump) prints this: PDOStatement Object ( [queryString] => SELECT * FROM animals )

但是如果我这样写:

foreach($results as $row) {
      echo $row['id'] . ' ' . $row['name'] . ' ' . $row['species'];
}

我的表格的每个内容都被打印出来.

every content of my table is printed.

所以 1) 如果我的对象中存在该信息，为什么 print_r 不显示任何信息?

So 1) why print_r doesn't show anything of that info if it exists inside my object?

和

2) 为什么我需要 $results->fetch() 或 $results->fetchAll() 来获取我已经从数据库并在结果集 $results 中?

2) why I would need $results->fetch() or $results->fetchAll() to fetch the info I have already fetched from the database and are in the result set $results?

推荐答案

因为 API 就是这样设计的，并且考虑到了灵活性.

Because that's how the API is designed, with flexibility in mind.

您的对象不包含所有数据集.相反，PDOStatement 实现了 迭代器/可遍历 接口，为每个 foreach 操作隐式调用 ->fetch.

Your object does not contain all datasets. Instead PDOStatement implements the Iterator/Traversable interface, implicitly calling ->fetch for each foreach operation.

然而，有些人更喜欢手动使用 ->fetch 或 ->fetchAll 一次提取所有行.

Some people however prefer to use ->fetch manually, or ->fetchAll for pulling all rows at once.

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