Practices regarding Nullable Foreign Keys & Alternatives to it - database

Question

I am currently working on designing and implementing a database.
We are running a conference, and there are two ways for attendees to register to the conference. The first is that the register and pay online - quite straightforward. The second is that they physically attend a "registration day" where they pay cash, and are added by one of our many admins. In this case, we would like to keep track of which admin added the attendee - for internal purposes.
We believe that it is best to store all attendees in the same table, and we would need a foreign key to track the admin that each attendee was added by. But for attendees registering online and paying themselves, we simply want to indicate that they were not added by any admin (so the foreign key would be Null).
Is the proposed idea of nullable foreign keys bad practice? Are there any benefits to storing attendees in different tables, where attendees registering online do not have a column for addedBy at all and cash-paying attendees have a non-nullable addedBy column?

Answer 1

I wouldn't store attendees in two (or more?) separate tables. They belong into the same table, and there should be a column which will tell you how someone got registered. Will it be "admin name" (or ID), checkbox, radio button item, whatever - just keep them in the same table.
Why? Because currently you think of two tables. It means that all code you write will have to be "duplicated" for those two tables - all inserts, updates, selects ... everything.
Now imagine that someone gets a super idea of the 3rd way of registration (snail mail, for example). What will you do then? Create the 3rd table, and triple code you wrote? That would be a really bad practice.
There's nothing "wrong" if a foreign key column doesn't contain any value, if you decide to do so - just document it. For example (Oracle syntax; would be similar in any other database):
SQL> create table admins
2 (id_admin number constraint pk_adm primary key,
3 name varchar2(20) not null
4 );
Table created.
SQL> create table attendees
2 (id_attendee number constraint pk_att primary key,
3 name varchar2(20) not null,
4 id_admin number constraint fk_att_adm references admins (id_admin)
5 );
Table created.
A few sample rows:
SQL> -- admin
SQL> insert into admins (id_admin, name)
2 select 1, 'Little' from dual;
1 row created.
SQL> -- attendees
SQL> -- registered online
SQL> insert into attendees (id_attendee, name) values (1, 'Mike');
1 row created.
SQL> -- registered on site
SQL> insert into attendees (id_attendee, name, id_admin) values (2, 'Scott', 1);
1 row created.
What do we know about them? It has to be outer join because - if it's not - you won't get info about people registered online:
SQL> select t.id_attendee, t.name attendee, a.name registered_by
2 from attendees t left join admins a on a.id_admin = t.id_admin;
ID_ATTENDEE ATTENDEE REGISTERED_BY
----------- -------------------- --------------------
1 Mike
2 Scott Little
SQL>
If you don't want to allow NULL values for a foreign key column, then a simple (and not bad at all) option is to create a "dummy" admin (let's call it "Online"):
SQL> insert into admins (id_admin, name) values (0, 'Online');
1 row created.
Set it for all users registered online (their id_admin column value is NULL):
SQL> update attendees set id_admin = 0 where id_admin is null;
1 row updated.
Disallow NULL values for a foreign key column:
SQL> alter table attendees modify id_admin not null;
Table altered.
Now you can use inner (instead of outer) join:
SQL> select t.id_attendee, t.name attendee, a.name registered_by
2 from attendees t join admins a on a.id_admin = t.id_admin;
ID_ATTENDEE ATTENDEE REGISTERED_BY
----------- -------------------- --------------------
1 Mike Online
2 Scott Little
SQL>
Snail mail? No problem, you don't have to modify any code you wrote so far:
SQL> insert into admins (id_admin, name) values (3, 'Snail mail');
1 row created.
SQL> insert into attendees (id_attendee, name, id_admin) values (3, 'King', 3);
1 row created.
SQL> select t.id_attendee, t.name attendee, a.name registered_by
2 from attendees t join admins a on a.id_admin = t.id_admin;
ID_ATTENDEE ATTENDEE REGISTERED_BY
----------- -------------------- --------------------
1 Mike Online
2 Scott Little
3 King Snail mail
SQL>