假设要为一个快递公司设计一个数据库。数据库能够查询寄件的客户和收件客户及相关信息 （姓名、地址、收件或寄件的时间）；有些客户 customer 可能既寄件又收件；由于每个包裹 packet 都需要追踪，所以需要存储包裹的历史位置 place（即可查看历史物流）。请画出该 应用的 ER 图，将其转换为关系模式，并指出相应的主码及外码。 答： customer(customer_id, name, address) Packet (packet_id, weight) Place (place_id, city, country, address) Sends (sender_id, receiver_id, packet_id, time_received, time_sent Foreign key sender_id references customer_id, foreign key receiver_id references customer_id, foreign key packet_id references Packet_id) Has_gone_through (packet_id, place_id, time foreign key packet_id references packet, foreign key place_id references place)