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.
mysql_select_db
(PHP 4, PHP 5)
mysql_select_db — Seleccionar una base de datos MySQL
Advertencia
Esta extensión fue declarada obsoleta en PHP 5.5.0 y eliminada en PHP 7.0.0. En su lugar debería utilzarse las extensiones MySQLi o PDO_MySQL. Véase también la guía MySQL: elegir una API. Las alternativas a esta función son:
- mysqli_select_db()
- PDO::__construct() (parte de dsn)
Descripción
Establece la base de datos activa actual en el servidor asociado con el identificador de enlace especificado. Cada llamada posterior a mysql_query() será ejecutada en la base de datos activa.
Parámetros
-
database_name -
El nombre de la base de datos que va a ser seleccionada.
-
link_identifier -
La conexión MySQL. Si no se especifica el identificador de enlace, se asume el último enlace abierto por mysql_connect(). Si no se encuentra este enlace, se intentará crear un nuevo enlace como si mysql_connect() hubiese sido invocada sin argumentos. Si no se encuentra o establece ninguna conexión, se genera un error de nivel
E_WARNING.
Valores devueltos
Devuelve true en caso de éxito o false en caso de error.
Ejemplos
Ejemplo #1 Ejemplo mysql_select_db()
<?php
$enlace = mysql_connect('localhost', 'usuario_mysql', 'contraseña_myql');
if (!$enlace) {
die('No se pudo conectar : ' . mysql_error());
}
// Hacer que foo sea la base de datos actual
$bd_seleccionada = mysql_select_db('foo', $enlace);
if (!$bd_seleccionada) {
die ('No se puede usar foo : ' . mysql_error());
}
?>Notas
Nota:
Por razones de compatibilidad con versiones anteriores, los siguientes alias obsoletos podrían usarse: mysql_selectdb()
Ver también
- mysql_connect() - Abre una conexión al servidor MySQL
- mysql_pconnect() - Abre una conexión persistente a un servidor MySQL
- mysql_query() - Enviar una consulta MySQL
add a note
User Contributed Notes 6 notes
james at gogo dot co dot nz ¶
20 years ago
Maarten ¶
19 years ago
About opening connections if the same parameters to mysql_connect() are used: this can be avoided by using the 'new_link' parameter to that function.
This parameter has been available since PHP 4.2.0 and allows you to open a new link even if the call uses the same parameters.
anotheruser at example dot com ¶
16 years ago
Cross-database join queries, expanding on Dan Ross's post...
Really, this is a mysql specific feature, but worth noting here. So long as the mysql user has been given the right permissions to all databases and tables where data is pulled from or pushed to, this will work. Though the mysql_select_db function selects one database, the mysql statement may reference another (the syntax for referencing a field in another db table being 'database.table.field').
<?php
$sql_statement = "SELECT
PostID,
AuthorID,
Users.tblUsers.Username
FROM tblPosts
LEFT JOIN Users.tblUsers ON AuthorID = Users.tblUsers.UserID
GROUP BY PostID,AuthorID,Username
";
$dblink = mysql_connect("somehost", "someuser", "password");
mysql_select_db("BlogPosts",$dblink);
$qry = mysql_query($sql_statement,$dblink);
?>
duncan at berrimans dot co dot uk ¶
12 years ago
Note that the manual is slightly misleading it states :-
"Sets the current active database on the server that's associated with the specified link identifier. Every subsequent call to mysql_query() will be made on the active database."
The 2nd statement is not true or at best unclear.
mysql_query() manual entry actually correctly states it will use the last link opened by mysql_connect() by default.
Thus if you have 2 connections you will need to specify the connection when calling mysql_query or issue the connect again to ensure the 1st database becomes the default, simply using mysql_select_db will not make the 1st database the default for subsequent calls to mysql_query.
Its probably only apparent when the two databases are on different servers.
miloshio at gmail dot com ¶
12 years ago
You can select MySQL database without using this function.
Simply right after connecting to MySQL
<?php $connection = mysql_connect("dabatbasehost", "username", "password"); ?>
perform this query:
<?php mysql_query("USE somedatabase", $connection); ?>
me at khurshid dot com ¶
17 years ago
Problem with connecting to multiple databases within the same server is that every time you do:
mysql_connect(host, username, passwd);
it will reuse 'Resource id' for every connection, which means you will end with only one connection reference to avoid that do:
mysql_connect(host, username, passwd, true);
keeps all connections separate.
