Extending SQL Table - sql-server

Question

I have 4 SQL tables: User, Student, Professor and Publication.
User has the common columns for any kind of user;
Student has columns specific for a student;
Professor has columns specific for a professor;
Publication is only for professors.
So I have:
create table dbo.[User] (
Id int identity not null
constraint PK_User_Id primary key clustered (Id),
-- Other user columns
)
create table dbo.Student (
UserId int not null
constraint PK_Student_UserId primary key clustered (Id),
-- Other student columns
)
create table dbo.Professor (
UserId int not null
constraint PK_Professor_Id primary key clustered (Id),
-- Other student columns
)
create table dbo.Publication (
Id int identity not null
constraint PK_Publication_Id primary key clustered (Id),
UserId int not null
-- Other student columns
)
alter table dbo.Student
add constraint FK_Student_UserId foreign key (UserId) references dbo.[User](Id);
alter table dbo.Professor
add constraint FK_Professor_UserId foreign key (UserId) references dbo.[User](Id);
alter table dbo.Publication
add constraint FK_Publication_UserId foreign key (UserId) references dbo.Professor(Id);
QUESTION
Should I have a column Id as PK in Professor and Student tables?
And make, for example, (Id, UserId) as the PK of Professor (Same for student)
Then Publication would reference Professor.Id and not Professor.UserId.
I am asking this because it sounds strange to have Publication to reference UserId from Professor table which can be confusing when I will have more tables.
Could someone please advice me on this?

Answer 1

In your current schema arrangement and without knowing your use cases (programmatically), one could make the argument that you don't need the Id identity columns for any of the extension tables. I assume this would be a 1 to 1 relationship to the User table anyway, so you'd at least want a unique constraint on the UserID columns, which you'd get by making it a PK anyway.

Answer 2

Things I like to consider are:
Can a professor ever become a different user ?
Is it possible for a professor to exist without an user ?
Is it possible for a single user to be two professors (multiple disciplines?)
If so, why wouldn't you give every professor an unique Id (ProfessorId), and only create a foreign key to the User table (UserId, you could call this UserFk).
In the publication table you can reference the professor by his/her id and call this one ProfessorFk. This way you create very efficient references between tables. The publication table than also gets a single PublicationId as primary key.
Correct me if i'm wrong, i don't know your use case. But it would seem reasonable that a professor can have multiple publications, but a publication can also be written by multiple professors ? This would mean you need an extra table between publication and professor for the n-n relationship.
About creating a professor key that is a combined key (Id, UserId). I personally dislike combined keys, if you want to reference this professor from your publication table you need two columns.
This would also suggest you can have multiple professors for the same user, if so, go for the single Id option.
This means i would create the following setup:
User Table
UserId
Student Table
StudentId
UserFk
Professor Table
ProfessorId
UserFk
ProfessorPublication Table
ProfessorFk
PublicationFk
Publication Table
PublicationId
So, it partly is based on what you want to be able to do with your data, and partly just your preference.