how make a plugin with the db connection

i have in this function that i would use like a plugin the connection to the db:

function rate($id) {

$dbhost = 'localhost';
$dbuser = 'user';
$dbpass = 'password';
$dbname = 'wordpress';

$conn = mysql_connect($dbhost, $dbuser, $dbpass) or die ('Error connecting to mysql');
mysql_select_db($dbname);

$tableName="test";

If i use this like a plugin is not necessary create the db connection, right ? And if the same plugin have another file that call the db, what i do ?