Null value for foreign key?

Question

SQL

There are two tables (schematic):
user
id - int not null pk
name - varchar(255) not null
title
id - int not null pk,
name - varchar(255) nit null
user_id - int
The title.user_id field refers to user.id as a foreign key. Question: Is null allowed in the title.user_id field?

Answer 1

Permissible. Don't forget to add ON DELETE SET NULL in the FK description.

Answer 2

For MS SQL - yes,
probably depends on the DBMS