thinkphp中使用group by时候如何用sum？

示例代码：
$reinfo=M("comreceiveinfo");
            $relist=$reinfo->where("userid='".$_SESSION["username"]."'")->select();
            $this->assign("relist",$relist);
            $shopcart=M("shoppingcart");
            $groupcart=$shopcart->field('supid,sum(shopprice) as sspp,sum(pv) as sspv')->where("agentd='".$_SESSION["username"]."'")->group("supid")->select();
            $pricetotal=0;
            $counttotal=0;
            $pvtotal=0;
            foreach($groupcart as $n=>$val){
                $groupcart[$n]["voo"]=$shopcart->where("supid=".$val["supid"])->select();
                $pricetotal=$pricetotal+$val["sspp"];
                $pvtotal=$pvtotal+$val["sspv"];
                $counttotal=$counttotal+count($groupcart[$n]["voo"]);
            }
            $this->assign("groupcart",$groupcart);
            $this->assign("pricetotal",$pricetotal);
            $this->assign("pvtotal",$pvtotal);
            $this->assign("counttotal",$counttotal);
            $this->display();