How do I select a MySQL database to use with PDO in PHP?

Question

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' );

Is there a PDO equivalent to mysqli::select_db?

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

EDIT

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.

Answer 1

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.

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");

Subsequent CREATE TABLE statements will be created in your newdatabase.

---

Re comment from @Mike:

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.

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));

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.

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

Answer 2

Alternatively, you can select a MySQL database to use after a PHP PDO object has already been created as below:

With USE STATEMENT. But remember here USE STATEMENT is mysql command

try
{
    $conn = new PDO("mysql:host=$servername;", $username, $password);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

    $conn->exec("use databasename");
    //application logic
}
catch(PDOException $e)
{
    echo $sql . "<br>" . $e->getMessage();
}

$conn = null;

I hope my code is helpful for requested

Answer 3

You should be setting the database when you create the PDO object. An example (from here)

<?php
$hostname = "localhost";
$username = "your_username";
$password = "your_password";

try {
    $dbh = new PDO("mysql:host=$hostname;dbname=mysql", $username, $password);
    echo "Connected to database"; // check for connection
    }
catch(PDOException $e)
    {
    echo $e->getMessage();
    }
?>

Answer 4

As far as I know, you have to create a new object for each connection. You can always extend the PDO class with a method which connects to multiple databases. And then use it as you like:

public function pickDatabase($db) {
  if($db == 'main') {
    return $this->db['main']; //instance of PDO object
  else
    return $this->db['secondary']; //another instance of PDO object
}

and use it like $yourclass->pickDatabase('main')->fetchAll('your stuff');