database - 解决数据库关系循环

Question

在数据库中，我有一个模型：

客户可以租车或出租汽车。如您所见，汽车有 OwnerID - 拥有汽车的客户，但同时客户也可以从其他所有者那里租用汽车，因此在订单表中他显示为用户。
那么我怎样才能改变模型来避免这样的循环，甚至有可能吗？

标签： databasedatabase-designforeign-keysdatabase-relations

Answer 1

考虑识别（存储在表格中）

• 所有者是谁以及他们拥有什么，以及
• 客户是谁。

.

create table persons (
  -- I prefer "people". YMMV.
  person_id integer primary key,
  person_name varchar(25) not null
  -- Other columns go here.
);

create table cars (
  -- VIN might be a better choice.
  car_id integer primary key
  -- Other columns go here.
);

create table car_owners (
  -- Only car owners known to us can rent out a car.
  car_id integer not null
    references cars(car_id),
  owner_id integer not null
    references persons (person_id),
  primary key (car_id, owner_id)
  -- Other columns go here.
);

create table customers (
  -- Any person can rent a car. (But some persons are not customers.)
  customer_id integer primary key  
    references persons (person_id)
  -- Other columns go here.
);

create table rentals (
  customer_id integer not null
    references customers (customer_id),
  car_id integer not null,
  owner_id integer not null,
  primary key (customer_id, car_id, owner_id),

  -- Don't rent a car unless we know who the owner is.
  foreign key (car_id, owner_id)                  
    references car_owners (car_id, owner_id)

  -- Other columns go here.
);