order mysql query result in decreasing order

I am trying to create a table for marks and rank team in decreasing order. i tried this.

$q = $db_con-> prepare("
SELECT @s:=@s+1 serial_number, t_name,name,marks FROM team,user,(SELECT @s:=0) AS s WHERE user.u_id=team.u_id ORDER BY marks DESC");

Above query displays result in DESC order of marks but serial numbers get altered. like..

sr no    team name   marks

 3          A         30
 1          B         20
 2          C         10

How to get serial numbers in order?