How to call an additional database

I would like to know how to call another MySQL non-WP database created outside my existing WP instance.

I’ve tried called the database in functions.php like this:


function db() {
$link = mysql_connect("X.SERVER.com","user","pw") or die("connection failed");
mysql_select_db("thedatabase") or die("database not found"); }
db();

The above function does enable to call the other database, but it shuts down the WP database call: none of my posts show up.

What is the proper way to call another database within WP?