Tp field 字段是可以添加函数的

$info = M('Order')->alias('a')
    ->field('count(DISTINCT(a.order_user_id)) as buy_user_num,count(*) as sum,sum(a.close_pay) as order_sum')
    ->join('__APP_TYPE_EDITION__ as b on b.type_id=a.type_id and b.edition_id=a.edition_id')
    ->join('__USER__ as c on c.user_id=a.order_user_id')
    ->where($where)
    ->order($order)
    ->find();
统计订单 的购买用户数，订单数，订单金额数