Select and count distinct active users by platform and compare if they exist in other tables
===

I have about six(6) tables each linked with userid. One of the tables is userinfo. The user info contains user details including their store platform(eg magento) 

 1. Userinfo contains both active and non-active users (active users have created at least one activity in the other 5 tables).
 2. I want to count distinct number of users in the userinfo with platform of magento who have records in any of the other tables. 
 3. Currently I am able to count distinct number of users in the other five tables with the ff code but want to join this with the userinfo table so I can select active users with platform magento. 
 4. Without adding the userinfo table means I have no way of selecting users by platform. 
 5. Selecting users in userinfo table only, with platform of magento will be easy, but that means I may select users who only register but do not go on to create activity on my app.

	 
             $query3 = ("SELECT   COUNT(*)
             FROM     (
               SELECT userid FROM table1
               UNION SELECT userid FROM table2
               UNION SELECT userid FROM table3
               UNION SELECT userid FROM table4
               UNION SELECT userid FROM table5
               ) AS UserIDs");
               $result3 = mysql_query($query3) or die(mysql_error()); 
               $row3 = mysql_fetch_row($result3);
               echo "Number of distinct users in all tables  = ".$row3[0] ."<br />";


          **Table 1**
          Id    userid    name   adresss
     
          **Table 2**
          Id  Title   Sex   userid
    
          **Table 3**
          Id   userid   amount
    
          **Table 4**
          Id  price  promotion  userid   productid
    
          **Table  5**
          Id  userid   category   tax   weight

          **userinfo**
          Id  userid   username   password   platform