Relational database and PHP: one-to-many relations with multiple one-tables - mysql

Question

Let’s assume there are some rows in a table cars, and each of these rows has an owner. If this owner were always a person (conveniently situated in a table persons), this would be your standard one-to-many relation.
However, what if the owner could not only be a person, but also a company (in a table companies)? How would this relationship be modeled and how would it be handled in PHP?
My first idea was to create a column person and a column company and check that one of them always stays NULL, while the other is filled – however, that seems somewhat inelegant and becomes impractical once there is a higher number of possible related tables.
My current assumption would be to not simply create the foreign key as an integer column person in the table, but to create a further table called tables, which gives IDs to the tables, and then split the foreign key into two integer columns: owner_table, containing the ID of the table (e.g. 0 for persons and 1 for companies), and owner_id, containing the owner ID.
Is this a viable and practical solution or is there some standard design pattern regarding such issues? Is there a name for this type of problem? And are there any PHP frameworks supporting such relations?
EDIT: Found a solution: Such structures are called polymorphic relations, and Laravel supports them.

Answer 1

There are multiple ways to do it.
You can go with two nullable foreign keys: one referencing company and the other user. Then you can have a check constraint which assure you one is null. With PostgreSQL:
CREATE TABLE car{
<your car fields>
company_id INT REFERENCES car,
person_id INT REFERENCES person,
CHECK(company_id IS NULL AND person_id IS NOT NULL
OR company_id IS NOT NULL AND person_id IS NULL)
};
Or you can use table inheritance (beware their limitations)
CREATE TABLE car_owner{
car_owner_id SERIAL
};
CREATE TABLE company{
<company fields>
} INHERITS(car_owner);
CREATE TABLE person{
<person fields>
} INHERITS(car_owner);
CREATE TABLE car{
<car fields>
car_owner_id INT REFERENCES car_owner
};