MYSQL On Duplicate (Exclusion Clause?)

[NuWeb]

$update = '('.$site_id.',"'.$user_id.'","'.$online_ip.'",'.$time.')';
$sql_online = 'INSERT INTO `user_online` (site_id, user_id, online_ip, online_timestamp) VALUES '.$update.' ON DUPLICATE KEY UPDATE online_timestamp=VALUES(online_timestamp)'; 

Table: user_online
online_ip - Primary Key
site_id - Unique Key
user_id
online_timestamp

As you can see the above works well as a Users Currently Online, as every time the user visits a page it inserts their information, or updates the timestamp.

The problem I have is that two websites useing same database, with the difference site_id value.

Ive noticed that If i go to Site 1, then Site 2. Only Site 1 is visitor is counted.

Is their a way around this obstacle, a conditional clause on site_id ?

[guido2004]

Make your primary key: online_ip, site_id

[NuWeb]

That seems to do the trick, thanks for your time!

It passed my mind that you could have two primary keys.

[r937]

strictly speaking, no, you can't have two primary keys

you can have only one primary key, but it can be a composite key consisting of more than one column