任何必需的可空外键的示例？ [英] Any example of a necessary nullable foreign key?

问题描述

Customers
 customer_id

Orders
 order_id
 customer_id fk

如果我有两个表，并在Orders表中的customer_id上定义一个外键，通过允许它为null我说我可以有一个顺序，没有与之相关的客户。因此，可空外键的概念似乎与外键的目的不一致，这是为了强制实施这个约束。

If I have two tables and define a foreign key on customer_id in the Orders table, by allowing it to be null I am saying that I can have an order that does not have a customer associated with it. As such, the notion of a nullable foreign key seems at odds with the purpose of a foreign key, which is to enforce this constraint.

有一个简单的例子需要可空外键的情况？

Is there a simple example of a situation in which a nullable foreign key would be necessary? Or an argument in favor of permitting them?

推荐答案

想象一个拥有团队TODO的表格。如果TODO尚未分配给团队成员，则其 user_id 为 NULL 。如果不是 NULL ，它是 users 表的外键。

Imagine a table that holds the TODOs of a team. If a TODO is not yet assigned to a member of the team, its user_id is NULL. If it is not NULL it is a foreign key to the users table.

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