Is there a way to create a unique constraint for 2 tables?

Question

Currently, I have the following 3 tables.

CREATE TABLE customer (
    id SERIAL PRIMARY KEY
);

CREATE TABLE google_subscription (
    fk_customer_id INTEGER NOT NULL UNIQUE,

    CONSTRAINT fk_customer_id_constraint
        FOREIGN KEY(fk_customer_id) 
        REFERENCES customer(id)
        ON DELETE RESTRICT
);

CREATE TABLE apple_subscription (
    fk_customer_id INTEGER NOT NULL UNIQUE,

    CONSTRAINT fk_customer_id_constraint
        FOREIGN KEY(fk_customer_id) 
        REFERENCES customer(id)
        ON DELETE RESTRICT
);

1. google_subscription is having fk_customer_id referencing to customer table id .
2. apple_subscription is having fk_customer_id referencing to customer table id .

I was wondering, is it ever possible to create a constraint, such that customer table id , will only be found in either google_subscription or apple_subscription , but NOT both?

Answer 1

No that is not possible. A constraint cannot span across multiple tables. Instead you can have another table subscriptions where you can have a unique index on customer(id) . Or you can have another column in customer table which will hold only 1 subscription at a time.