How can I a foreign key to NULL in SQL Server database?

Question

I'd like to get your advice about how i can add foreign key constraint with set null.

I wrote this SQL:

ALTER TABLE DEPARTMENT
ADD CONSTRAINT DEPARTMENT_HIDEPTID_FK 
    FOREIGN KEY (HIDEPTID) REFERENCES DEPARTMENT(DEPTID) 
    SET NULL

but, at the end of the above statement, set null causes an error.

I can add just foreign key, but with set null, I cannot.

Thank you.

Answer 1

Thank you for your hint.

ALTER TABLE DEPARTMENT
    ADD CONSTRAINT DEPARTMENT_HIDEPTID_FK 
        FOREIGN KEY(HIDEPTID) REFERENCES DEPARTMENT(DEPTID) 
            ON DELETE CASCADE

This works.

The reason I'd like to set null at foreign key is that the table references itself.

Thank you.