How to get unique numbers of authors through all forums

Hi,

In the stats, you can have the unique number of author per forum, but not for the whole forums.

I did some code that is working fine, but that simply add the number of different authors per forum, but not really the number of unique authors.
Any idea on how to do it ?

Thanks

$sql="Select distinct(table_name) from ".$pho_main." where folder='0'";
$q=new query($DB, $sql);
$forums=$q->getrow();

$totalauthors=0;
while(is_array($forums)){
//Count authors in each forum
$sql="SELECT DISTINCT author,email FROM $forums[table_name] WHERE datestamp >='$date_range'";
$query=new query($DB, $sql);
$authors=$query->numrows();
//Add Them To totals
$totalauthors=$totalauthors+$authors;
$forums=$q->getrow();
}

you'd have to make them unique through all the forums.
all you do is summing up the unique authors you got for each forum.