mysql sum的多條件求和

1.無條件求和

   sum(字段名)；

2.多條件求和

   sum(IF(c1>0, c1, 0));

  (IF(o.is_cod=0,o.order_amount-IF(b.back_status>0 AND b.back_status<5, b.refund_money, 0),0))-SUM(IF(o.is_cod=0,o.shipping_fee, 0))-SUM(IF(o.change_amount=0,o.change_amount, 0))