如何在 PHP 中选择要与 PDO 一起使用的 MySQL 数据库?

问题描述

我想在创建 PHP PDO 对象后选择要使用的 MySQL 数据库.我该怎么做?

I want to select a MySQL database to use after a PHP PDO object has already been created. How do I do this?

// create PDO object and connect to MySQL
$dbh = new PDO( 'mysql:host=localhost;', 'name', 'pass' );

// create a database named 'database_name'

// select the database we just created ( this does not work )
$dbh->select_db( 'database_name' );

是否有等效于 mysqli::select_db 的 PDO?

Is there a PDO equivalent to mysqli::select_db?

也许我试图不正确地使用 PDO?请帮助或解释.

Perhaps I'm trying to use PDO improperly? Please help or explain.

编辑

我不应该使用 PDO 创建新数据库吗?我知道使用 PDO 的大部分好处在一个很少使用的操作中丢失了，它不会像 CREATE DATABASE 那样插入数据，但是不得不使用不同的连接来创建数据库似乎很奇怪，然后创建 PDO 连接以进行其他调用.

Should I not be using PDO to create new databases? I understand that the majority of benefits from using PDO are lost on a rarely used operation that does not insert data like CREATE DATABASE, but it seems strange to have to use a different connection to create the database, then create a PDO connection to make other calls.

推荐答案

通常，您会在连接时在 DSN 中指定数据库.但是，如果您正在创建一个新数据库，显然您不能在创建之前将该数据库指定为 DSN.

Typically you would specify the database in the DSN when you connect. But if you're creating a new database, obviously you can't specify that database the DSN before you create it.

您可以使用 USE 语句更改默认数据库:

You can change your default database with the USE statement:

$dbh = new PDO("mysql:host=...;dbname=mysql", ...);

$dbh->query("create database newdatabase");

$dbh->query("use newdatabase");

随后的 CREATE TABLE 语句将在您的新数据库中创建.

Subsequent CREATE TABLE statements will be created in your newdatabase.

来自@Mike 的重新评论:

Re comment from @Mike:

当您像这样切换数据库时，它似乎会强制 PDO 模拟准备好的语句.将 PDO::ATTR_EMULATE_PREPARES 设置为 false，然后尝试使用其他数据库将失败.

When you switch databases like that it appears to force PDO to emulate prepared statements. Setting PDO::ATTR_EMULATE_PREPARES to false and then trying to use another database will fail.

我只是做了一些测试，但我没有看到这种情况发生.更改数据库只发生在服务器上，它不会更改客户端中 PDO 配置的任何内容.举个例子:

I just did some tests and I don't see that happening. Changing the database only happens on the server, and it does not change anything about PDO's configuration in the client. Here's an example:

<?php

// connect to database
try {
    $pdo = new PDO('mysql:host=huey;dbname=test', 'root', 'root');
    $pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $pdo->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
} catch(PDOException $err) {
    die($err->getMessage());
}

$stmt = $pdo->prepare("select * from foo WHERE i = :i");
$result = $stmt->execute(array("i"=>123));
print_r($stmt->fetchAll(PDO::FETCH_ASSOC));

$pdo->exec("use test2");

$stmt = $pdo->prepare("select * from foo2 WHERE i = :i AND i = :i");
$result = $stmt->execute(array("i"=>456));
print_r($stmt->fetchAll(PDO::FETCH_ASSOC));

如果您说的是真的，那么这应该可以正常工作.仅当 PDO::ATTR_EMULATE_PREPARES 为真时，PDO 才能多次使用给定的命名参数.因此，如果您说这个属性设置为 true 作为更改数据库的副作用，那么它应该可以工作.

If what you're saying is true, then this should work without error. PDO can use a given named parameter more than once only if PDO::ATTR_EMULATE_PREPARES is true. So if you're saying that this attribute is set to true as a side effect of changing databases, then it should work.

但它不起作用——它得到一个错误参数编号无效"，表明非模拟的准备好的语句仍然有效.

But it doesn't work -- it gets an error "Invalid parameter number" which indicates that non-emulated prepared statements remains in effect.

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