PHP: mysql_select_db

User Contributed Notes
mysql_select_db

buzz at oska dot com
06-May-2005 07:39


As has been already commented, opening multiple connection handles with:

<?php 

$connection_handle =  mysql_connect($hostname_and_port,$user,$password);

?>

causes the connection ID/handle to be REUSED if the exact same parameters are passed in to it.   This can be annoying if you want to work with multiple databases on the same server, but don't want to (a) use the database.table syntax in all your queries or (b) call the mysql_select_db($database) before every SQL query just to be sure which database you are working with.     

My solution is to create a handle for each database with mysql_connect (using ever so slightly different connection properties), and assign each of them to their own database permanently.  each time I do a mysql_query(...) call, I just include the connection handle that I want to do this call on eg (ive left out all error checking for simplicity sake):

<?php

// none of thesehandles are re-used as the connection parameters are different on them all, despite connecting to the same server (assuming 'myuser' and 'otheruser' have the same privileges/accesses in mysql)

$handle_db1 = mysql_connect("localhost","myuser","apasswd");

$handle_db2 = mysql_connect("127.0.0.1","myuser","apasswd");

$handle_db3 = mysql_connect("localhost:3306","myuser","apasswd");

$handle_db4 = mysql_connect("localhost","otheruser","apasswd");



// give each handle it's own database to work with, permanently.

mysql_select_db("db1",$handle_db1);

mysql_select_db("db2",$handle_db2);

mysql_select_db("db3",$handle_db3);

mysql_select_db("db4",$handle_db4);



//do a query from db1:

$query = "select * from test"; $which = $handle_db1;

mysql_query($query,$which);



//do a query from db2 :

$query = "select * from test"; $which = $handle_db2;

mysql_query($query,$which);



//etc 

?>



Note that we didn't do a mysql_select_db between queries , and we didn't use the database name in the query either.



Of course, it has the overhead of setting up an extra connection.... but you may find this is preferable in some cases...

Dan Ross
12-Feb-2004 03:43


Another way to select from 2 different databases on the same server:



mysql_select_db("db1");



$res_db1 = mysql_query("select * from db1.foobar");

$res_db2 = mysql_query("select * from db2.foobar");



I.e. just prepend database name.

james at gogo dot co dot nz
16-Jan-2004 06:45


Be carefull if you are using two databases on the same server at the same time.  By default mysql_connect returns the same connection ID for multiple calls with the same server parameters, which means if you do 



<?php

  $db1 = mysql_connect(...stuff...);

  $db2 = mysql_connect(...stuff...);

  mysql_select_db('db1', $db1);

  mysql_select_db('db2', $db2); 

?>



then $db1 will actually have selected the database 'db2', because the second call to mysql_connect just returned the already opened connection ID !



You have two options here, eiher you have to call mysql_select_db before each query you do, or if you're using php4.2+ there is a parameter to mysql_connect to force the creation of a new link.

doug at xamo dot com
17-Dec-2003 02:39


When you need to query data from multiple databases, note that mysql_select_db("db2")  doesn't prevent you from fetching more rows with result sets returned from "db1".



mysql_select_db("db1");



$res_db1=mysql_query("select * from foobar");



myqsl_select_db("db2);



$row_db1=mysql_fetch_object($res_db1);



$res_db2=mysql_query("select * from test where id='$row_db1->id'");

mysql_select_db

Description

Parameters

Return Values

Examples

Notes

See Also