任何必要的可空外键示例? [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，允许它为空，我是说我可以有一个没有客户关联的订单.因此，可空外键的概念似乎与外键的目的不一致，即强制执行此约束.

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.

是否有一个简单的例子说明需要可空外键的情况?还是赞成允许他们的论据?

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

想象一个包含团队 TODO 的表格.如果 TODO 尚未分配给团队成员，则其 user_id 为 NULL.如果它不是 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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