Question

Why foreign keys are allowed to have NULL values? Explain with an example.

Answer 1

The value of the foreign key is derived from the Primary key of the primary table. Sometimes, we want to enter a record, which is not related. To handle this situation, it is allowed to enter a NULL value in the foreign key.

For example, Suppose a shopkeeper wants to sell a product, but the customer is not a regular customer, so his customer id does not exist. In this case, a NULL value is entered into the customerId field.