Any example of a necessary nullable foreign key?

Question

Customers
 customer_id

Orders
 order_id
 customer_id fk

If I have two tables and define a foreign key on customer_id in the Orders table, by allow

Answer 1

There is another situation that I can think of:

Within one table, you have an ID field that uniquely identifies that record. That same record may (or may not) have a single parent.

Table with the following columns:

• id as an integer, auto-increment, not nullable
• parentid as an integer, nullable.

With a foreign key constraint, you can ensure that if the field is not null, then it refers to a valid parent. Furthermore, if you attempt to delete the parent record without fixing the children, then the parent cannot be deleted.