Foreign Keys are great!

Too bad they were not invented until SQL-89, and could thus not be considered when the JOIN syntax we all know was designed and formalized by ANSI in SQL-86.

Imagine if we could simply give the name of a foreign key,
as an argument to JOIN, letting the query planner look up the foreign key columns, to do the join on.

The default names for foreign keys are not very user-friendly, due to the SQL standard unfortunate decision to require the foreign key name to be unique per namespace, and not just per table, which would have made more sense.

In PostgreSQL, foreign keys just need to be unique per table, so there we could give them names such as "customer" instead of "rental_customer_id_fkey".

Imagine if we instead of:

SELECT
language.name,
COUNT(*)
FROM rental
JOIN customer
ON customer.customer_id = rental.customer_id
JOIN address
ON address.address_id = customer.address_id
JOIN city
ON city.city_id = address.city_id
JOIN country
ON country.country_id = city.country_id
JOIN inventory
ON inventory.inventory_id = rental.inventory_id
JOIN film
ON film.film_id = inventory.film_id
JOIN language
ON language.language_id = film.language_id
WHERE country.country = 'Sweden'
GROUP BY language.name

Would write something like:

SELECT
language.name,
COUNT(*)
FROM rental
JOIN rental->rental_customer_id_fkey AS customer
JOIN customer->customer_address_id_fkey AS address
JOIN address->address_city_id_fkey AS city
JOIN city->city_country_id_fkey AS country
JOIN rental->rental_inventory_id_fkey AS inventory
JOIN inventory->inventory_film_id_fkey AS film
JOIN film->film_language_id_fkey AS language
WHERE country.country = 'Sweden'
GROUP BY language.name

After renaming the foreign keys:

ALTER TABLE rental RENAME CONSTRAINT rental_customer_id_fkey TO customer;
ALTER TABLE rental RENAME CONSTRAINT rental_inventory_id_fkey TO inventory;
ALTER TABLE customer RENAME CONSTRAINT customer_address_id_fkey TO address;
ALTER TABLE address RENAME CONSTRAINT address_city_id_fkey TO city;
ALTER TABLE city RENAME CONSTRAINT city_country_id_fkey TO country;
ALTER TABLE inventory RENAME CONSTRAINT inventory_film_id_fkey TO film;
ALTER TABLE film RENAME CONSTRAINT film_language_id_fkey TO language;

We could write this as:

SELECT
language.name,
COUNT(*)
FROM rental
JOIN rental->customer
JOIN customer->address
JOIN address->city
JOIN city->country
JOIN rental->inventory
JOIN inventory->film
JOIN film->language
WHERE country.country = 'Sweden'
GROUP BY language.name

And if allowing such a "foreign key operator" to be chained,
we could write:

SELECT
language.name,
COUNT(*)
FROM rental
JOIN rental->customer->address->city->country
JOIN rental->inventory->film->language
WHERE country.country = 'Sweden'
GROUP BY language.name

This is similar to "4.9 Reference types" in the SQL standard ISO/IEC 9075-2:2016(E), but it wouldn't require a separate REF column, it would merely use the existing foreign keys which we already have in well designed proper database schemas. We would just need to give them better names.

Q: How would we name foreign keys if there are two going to the same table?

A: Imagine having a "users" table with two columns "child_user_id" and "parent_user_id", both referencing "users". The foreign keys on such columns could simply be named "child" and "parent".