check for the uniqueness of an entry !?

Hi, how can I check if a user_id is already being used by another user and automatically tell the user that the id he/she requested is already in use (user_id is PRIMARY KEY and UNIQUE ID in a mysql table)
Thanks

------------------

$result=mysql_query("select * from usertable where userid='$userid'");
$exists=mysql_num_rows($result);
if ($exists)
{
//username already used
}
else
{
//username not used
}

Faster way

$result=mysql_query("select count(*) from usertable where userid='$userid'");
$number = mysql_fetch_row($result);
if ($number[0]>='1')
{
//username already used
}
else
{
//username not used
}

Yes and no.

Using count(*) would be faster when there is actual data returned i.e. userid is used AND the data returned is large. The fastest way would be to return a single field that contains an integer (the smaller the better) or a char(n) field where n is small (less than 5 characters). This reduces the overhead of returning large data and the overhead of mysql performing the count() function.

Using a boolean conditional is faster than a comparison.

The num_rows function is quicker than fetch_row because it returns an integer, not an array (doesn't matter that the array is one element and that element is an integer.)