空外键(Null Foreign Key)

空外键(Null Foreign Key)

新手程序员在这里。

我有3个表，即产品，类别和子类别。 我以这种方式配置他们的关系：

产品到类别：多对多 产品到子类别：一对一 分类的子类别：多对一

我添加了一个subcategory_id列，它是产品表中的外键（用于映射产品和子类别表）。 如果产品具有子类别，则此方法有效。 现在的问题是我的产品没有子类别。 假设是subcategory_id列将为null，但不允许。 这有解决方法吗？ 我也认为关系配置与此有关。 有什么想法吗？

newbie programmer here.

I have 3 tables namely product, category, and subcategory. I configured their relationships this way:

Product to Category: Many-to-many Product to Subcategory: One-to-one Subcategory to Category: Many-to-one

I added a subcategory_id column which is a foreign key in the product table (for mapping the product and subcategory tables). This works if a product has a subcategory. Now the problem is I have products which doesn't have subcategory. Supposed to be the subcategory_id column will be null, but it's not allowed. Is there a workaround for this? I also think that the relationship configuration has something to do with this. Any thoughts?

最满意答案

只要subcategory_id可以为空，您应该能够添加一个外键，该外键将在列不为null的所有情况下强制执行关系的完整性。 这是一个相当常见的用例。

As long as the subcategory_id is nullable, you should be able to add a foreign key that will enforce the integrity of the relationship in all cases where the column is not null. This is a fairly common use case.