SQL group by 遇到的坑

背景介绍：

有两张表：busi_customer用于保存客户信息；busi_order用于保存客户的订单信息；

建表语句如下：

CREATE TABLE "public"."busi_customer" (
  "id" int8 NOT NULL DEFAULT nextval('busi_customer_id_seq'::regclass),
  "cust_code" varchar(255) COLLATE "pg_catalog"."default",
  "cust_name" varchar(255) COLLATE "pg_catalog"."default",
  CONSTRAINT "busi_customer_pkey" PRIMARY KEY ("id")
); 

CREATE TABLE "public"."busi_order" (
  "id" int8 NOT NULL DEFAULT nextval('busi_order_id_seq'::regclass),
  "cust_code" varchar(255) COLLATE "pg_catalog"."default" NOT NULL,
  "amount" numeric(10,2) NOT NULL,
  "order_number" text COLLATE "pg_catalog"."default",
  CONSTRAINT "busi_order_pkey" PRIMARY KEY ("id")
); 

需求：请统计不同用户的累计订单金额。

两张表中分别插入数据：

busi_customer

busi_order

 统计SQL：

select o.cust_code,sum(amount) as amonut 
from busi_order o 
left join 
busi_customer c on o.cust_code = c.cust_code
group by o.cust_code;

统计结果：

现在在busi_customer表中插入一条cust_code为A001的数据后，用户表的结果如下：

再执行统计SQL，发现A001的订单金额会重复计算：