How do you connect to multiple MySQL databases on a single webpage?

Question

I have information spread out across a few databases and want to put all the information onto one webpage using PHP. I was wondering how I can connect to multiple databases

Answer 1

You might be able to use MySQLi syntax, which would allow you to handle it better.

Define the database connections, then whenever you want to query one of the database, specify the right connection.

E.g.:

$Db1 = new mysqli('$DB_HOST','USERNAME','PASSWORD'); // 1st database connection 
$Db2 = new mysqli('$DB_HOST','USERNAME','PASSWORD'); // 2nd database connection

Then to query them on the same page, use something like:

$query = $Db1->query("select * from tablename")
$query2 = $Db2->query("select * from tablename")
die("$Db1->error");

Changing to MySQLi in this way will help you.

Answer 2

Instead of mysql_connect use mysqli_connect.

mysqli is provide a functionality for connect multiple database at a time.

$Db1 = new mysqli($hostname,$username,$password,$db_name1); 
// this is connection 1 for DB 1

$Db2 = new mysqli($hostname,$username,$password,$db_name2); 
// this is connection 2 for DB 2

Answer 3

You don't actually need select_db. You can send a query to two databases at the same time. First, give a grant to DB1 to select from DB2 by GRANT select ON DB2.* TO DB1@localhost;. Then, FLUSH PRIVILEGES;. Finally, you are able to do 'multiple-database query' like SELECT DB1.TABLE1.id, DB2.TABLE1.username FROM DB1,DB2 etc. (Don't forget that you need 'root' access to use grant command)

Answer 4

Warning : mysql_xx functions are deprecated since php 5.5 and removed since php 7.0 (see http://php.net/manual/intro.mysql.php), use mysqli_xx functions or see the answer below from @Troelskn

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You can make multiple calls to mysql_connect(), but if the parameters are the same you need to pass true for the '$new_link' (fourth) parameter, otherwise the same connection is reused. For example:

$dbh1 = mysql_connect($hostname, $username, $password); 
$dbh2 = mysql_connect($hostname, $username, $password, true); 

mysql_select_db('database1', $dbh1);
mysql_select_db('database2', $dbh2);

Then to query database 1 pass the first link identifier:

mysql_query('select * from tablename', $dbh1);

and for database 2 pass the second:

mysql_query('select * from tablename', $dbh2);

If you do not pass a link identifier then the last connection created is used (in this case the one represented by $dbh2) e.g.:

mysql_query('select * from tablename');

Other options

If the MySQL user has access to both databases and they are on the same host (i.e. both DBs are accessible from the same connection) you could:

• Keep one connection open and call mysql_select_db() to swap between as necessary. I am not sure this is a clean solution and you could end up querying the wrong database.
• Specify the database name when you reference tables within your queries (e.g. SELECT * FROM database2.tablename). This is likely to be a pain to implement.

Also please read troelskn's answer because that is a better approach if you are able to use PDO rather than the older extensions.

Answer 5

Unless you really need to have more than one instance of a PDO object in play, consider the following:

$con = new PDO('mysql:host=localhost', $username, $password, 
      array(PDO::ATTR_PERSISTENT => true));

Notice the absence of dbname= in the construction arguments.

When you connect to MySQL via a terminal or other tool, the database name is not needed off the bat. You can switch between databases by using the USE dbname statement via the PDO::exec() method.

$con->exec("USE someDatabase");
$con->exec("USE anotherDatabase");

Of course you may want to wrap this in a catch try statement.

Answer 6

If you use PHP5 (And you should, given that PHP4 has been deprecated), you should use PDO, since this is slowly becoming the new standard. One (very) important benefit of PDO, is that it supports bound parameters, which makes for much more secure code.

You would connect through PDO, like this:

try {
  $db = new PDO('mysql:dbname=databasename;host=127.0.0.1', 'username', 'password');
} catch (PDOException $ex) {
  echo 'Connection failed: ' . $ex->getMessage();
}

(Of course replace databasename, username and password above)

You can then query the database like this:

$result = $db->query("select * from tablename");
foreach ($result as $row) {
  echo $row['foo'] . "\n";
}

Or, if you have variables:

$stmt = $db->prepare("select * from tablename where id = :id");
$stmt->execute(array(':id' => 42));
$row = $stmt->fetch();

If you need multiple connections open at once, you can simply create multiple instances of PDO:

try {
  $db1 = new PDO('mysql:dbname=databas1;host=127.0.0.1', 'username', 'password');
  $db2 = new PDO('mysql:dbname=databas2;host=127.0.0.1', 'username', 'password');
} catch (PDOException $ex) {
  echo 'Connection failed: ' . $ex->getMessage();
}