计算并列出某一行中的所有人

Okay I'm trying to count the amount of people in a clan. I have all the clans sent in a database like this: name(clan name) roomOwner(Owner of clan) I have a user table like this: username, clan(Owner of Clan) How could I get all of the people who have the same clan and count that? I figure it would be something like this but I'm unsure atm

    function countMembers($clan) {
    include "mysql.ws";
    $query = mysql_query("SELECT u.clan AS clan, a.*
FROM
  users u
  JOIN clans a ON(a.roomOwner = u.clan)
WHERE
 u.username = '$clan'");
    while($row = mysql_num_rows($query)){
        $members = count($row);
            return $row;
    }
}

This should work:

select clan, count(username) as "# Of Members" 
from users 
where clan is not null
group by clan