connection to database doesn't work while using normal mysql codes !

hello there ,
I have something that I just notice ..
I know that wordpress uses the codes located in ‘wp-config.php‘ file , to connect to a specific database ,,
the codes for specifying the host , database , user and password :
define(‘DB_NAME’, ‘db’);
define(‘DB_USER’, ‘user’);
define(‘DB_PASSWORD’, ‘pass’);
define(‘DB_HOST’, ‘localhost’);

But I just notice something ,that when I use some mysql codes in a wordpress site it doesn’t work because there is no connection to any server or any database .
for example in single.php , Im using this code :

$myrows = mysql_query("SELECT * FROM wp_slides_plugin WHERE slide_name='Main' ORDER BY position ASC"); // order by

while($row= mysql_fetch_array($myrows)){
$url[]=$row['url'];
$title[]=$row['title'];
$link[]=$row['link'];
}

It doesnt work unless I connect the site with the database I use , so I have to add the line :
mysql_query("use db", mysql_connect('localhost','user','pass') );

so ,I have to connect to the database twice , its not enough to use the wordpress codes for connection !!

do I have to add mysql connection line every time I want to use mysql ?
or there is some way to fix this problem ?

Note: This problem starts when I start using ‘MySQL “Improved” extension‘ in order to use MySQLi on easyapache . everything was working normally with wordpress sites before enable ‘MySQL “Improved” extension‘ .
and as you can see , after enable that choice , all my wordpress sites stop reading the connection codes of wordpress while using mysql codes .

Please help me if you can ..