Can Foreign Keys be Null?

Question

Hello,

According to me, FKs can't be null as otherwise It will violate the Foreign Key constraint.

FK refers to primary key and Primary key can't be null.

Can you help me understand this better?

Answer 1

FK refers to primary key and Primary key can't be null.

Primary key value never can be NULL, that's correct.

But a foreign key column can be NULL(able), it then references no PK and that's completley valid.

Answer 2

Hi @sonal khatri

It will violate the Foreign Key constraint.

Primary Key Constraints and Foreign Key Constraints are two different things. Foreign key simply requires that the value in that field must exist first in a different table (the parent table).

NULL by definition is not a value. Null means that we do not yet know what the value is.

Here is a simple example: Suppose we have an employee table with employee_id and manager_id where manager_id is foreign key and CEO of the organization doesn't have manager so his manager_id will be NULL in Employee table.

Best regards,

Cosmog Hong

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