Question

What is the best way to emulate Tagged union in databases?
I’m talking about something like this:

create table t1 {
  vehicle_id INTEGER NOT NULL REFERENCES car(id) OR motor(id) -- not valid
  ...
}

where vehicle_id would be id in car table OR motor table, and it would know which.

(assume that motor and car tables have nothing in common0

Answer 1

Some people use a design called Polymorphic Associations to do this, allowing vehicle_id to contain a value that exists either in car or motor tables. Then add a vehicle_type that names the table which the given row in t1 references.

The trouble is that you can’t declare a real SQL foreign key constraint if you do this. There’s no support in SQL for a foreign key that has multiple reference targets. There are other problems, too, but the lack of referential integrity is already a deal-breaker.

A better design is to borrow a concept from OO design of a common supertype of both car and motor:

CREATE TABLE Identifiable (
 id SERIAL PRIMARY KEY
);

Then make t1 reference this super-type table:

CREATE TABLE t1 (
  vehicle_id INTEGER NOT NULL,
  FOREIGN KEY (vehicle_id) REFERENCES identifiable(id)
  ...
);

And also make the sub-types reference their parent supertype. Note that the primary key of the sub-types is not auto-incrementing. The parent supertype takes care of allocating a new id value, and the children only reference that value.

CREATE TABLE car (
  id INTEGER NOT NULL,
  FOREIGN KEY (id) REFERENCES identifiable(id)
  ...
);

CREATE TABLE motor (
  id INTEGER NOT NULL,
  FOREIGN KEY (id) REFERENCES identifiable(id)
  ...
);

Now you can have true referential integrity, but also support multiple subtype tables with their own attributes.

---

The answer by @Quassnoi also shows a method to enforce disjoint subtypes. That is, you want to prevent both car and motor from referencing the same row in their parent supertype table. When I do this, I use a single-column primary key for Identifiable.id but also declare a UNIQUE key over Identifiable.(id, type). The foreign keys in car and motor can reference the two-column unique key instead of the primary key.