問(wèn)題描述

我想在創(chuàng)建 PHP PDO 對(duì)象后選擇要使用的 MySQL 數(shù)據(jù)庫(kù).我該怎么做?

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?

也許我試圖不正確地使用 PDO?請(qǐng)幫助或解釋.

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

編輯

我不應(yīng)該使用 PDO 創(chuàng)建新數(shù)據(jù)庫(kù)嗎?我知道使用 PDO 的大部分好處在一個(gè)很少使用的操作中丟失了，它不會(huì)像 CREATE DATABASE 那樣插入數(shù)據(jù)，但是不得不使用不同的連接來(lái)創(chuàng)建數(shù)據(jù)庫(kù)似乎很奇怪，然后創(chuàng)建 PDO 連接以進(jìn)行其他調(diào)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.

推薦答案

通常，您會(huì)在連接時(shí)在 DSN 中指定數(shù)據(jù)庫(kù).但是，如果您正在創(chuàng)建一個(gè)新數(shù)據(jù)庫(kù)，顯然您不能在創(chuàng)建之前將該數(shù)據(jù)庫(kù)指定為 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 語(yǔ)句更改默認(rèn)數(shù)據(jù)庫(kù):

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");

隨后的 CREATE TABLE 語(yǔ)句將在您的新數(shù)據(jù)庫(kù)中創(chuàng)建.

Subsequent CREATE TABLE statements will be created in your newdatabase.

來(lái)自@Mike 的重新評(píng)論:

Re comment from @Mike:

當(dāng)您像這樣切換數(shù)據(jù)庫(kù)時(shí)，它似乎會(huì)強(qiáng)制 PDO 模擬準(zhǔn)備好的語(yǔ)句.將 PDO::ATTR_EMULATE_PREPARES 設(shè)置為 false，然后嘗試使用其他數(shù)據(jù)庫(kù)將失敗.

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.

我只是做了一些測(cè)試，但我沒(méi)有看到這種情況發(fā)生.更改數(shù)據(jù)庫(kù)只發(fā)生在服務(wù)器上，它不會(huì)更改客戶端中 PDO 配置的任何內(nèi)容.舉個(gè)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));

如果您說(shuō)的是真的，那么這應(yīng)該可以正常工作.僅當(dāng) PDO::ATTR_EMULATE_PREPARES 為真時(shí)，PDO 才能多次使用給定的命名參數(shù).因此，如果您說(shuō)這個(gè)屬性設(shè)置為 true 作為更改數(shù)據(jù)庫(kù)的副作用，那么它應(yīng)該可以工作.

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——它得到一個(gè)錯(cuò)誤參數(shù)編號(hào)無(wú)效"，表明非模擬的準(zhǔn)備好的語(yǔ)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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