外键如何具有空值，而主键不能有？ [英] How foreign key can have null values where as primary key can't have?

问题描述

我很惊讶外键可以在sql server中引用其引用表上的空值。如果有人用简单的例子向我解释，我会很感激。

谢谢

I am surprise of that foreign key can refer to null value on its reference table in sql server.I would appreciate if someone explain this to me with simple example.
Thank you

推荐答案

为什么他们不应该有 null 值？主键不能，因为 null 不是唯一值 - 这是主键 [ ^ ]。 外键 [ ^ ]只是将一个表中的一行与另一个表中的唯一行相关联，因此当没有时， null 这样的行存在。

Why shouldn't they have null values? Primary keys can't because null is not a unique value - which is a requirement of a Primary key[^]. A Foreign key[^] just relates a row in one table to a unique row in another table, and so can be null when no such row exists.

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