Postgresql – Two-column foreign key constraint only when third column is NOT NULL

constraintdatabase-designforeign keynormalizationpostgresql

Given the following tables:

CREATE TABLE verified_name (
  id               SERIAL PRIMARY KEY,
  name             TEXT NOT NULL,
  email            TEXT NOT NULL,
  UNIQUE (name, email)
);

CREATE TABLE address (
  id               SERIAL PRIMARY KEY,
  name             TEXT NOT NULL,
  email            TEXT NOT NULL,
  verified_name_id INTEGER NULL REFERENCES verified_name(id)
);

How can I add an additional constraint that when address.verified_name_id is not NULL, the name and email fields on the address must match the those on the referenced verified_name?

I've tried adding the following to address:

FOREIGN KEY (name, email) REFERENCES verified_name(name, email)

…but that constraint is being applied even when verified_name_id is NULL.

I'm looking for something similar to the partial index syntax with a clause like WHERE verified_name_id IS NOT NULL, but simply appending a clause like that to the FOREIGN KEY constraint doesn't work.

Current undesirable solution:

I can add the following constraints to verified_name:

UNIQUE (name, email),
UNIQUE (id, name, email)

And the following constraint to address:

FOREIGN KEY (verified_name_id, name, email) REFERENCES verified_name(id, name, email)

…but that creates an extra constraint on verified_name that I'd prefer not to have (it's a valid logical constraint, but it's also superfluous and there are minor performance implications).

Best Answer

Proper solution

The core of the problem is the data model. In a normalized schema, you wouldn't store name and email redundantly. Could look like this:

CREATE TABLE name (
  name_id          SERIAL PRIMARY KEY,
  name             TEXT NOT NULL,
  email            TEXT NOT NULL,
  verified         BOOLEAN NOT NULL DEFAULT FALSE,
  UNIQUE (name, email)
);

CREATE TABLE address (
  address_id       SERIAL PRIMARY KEY,
  name_id          INT REFERENCES name(name_id)
  ...
);

If not-yet verified names should be allowed to break the UNIQUE constraint, you can replace it with a partial UNIQUE INDEX (much like you had in mind):

CREATE UNIQUE INDEX name_verified_idx ON name(name, email) WHERE verified;

Work with what you have

While stuck with your unfortunate design, the solution you already found yourself fits your requirements perfectly. A FOREIGN KEY with the default MATCH SIMPLE behavior matches the

partial index syntax with a clause like WHERE verified_name_id IS NOT NULL.

Quoting the manual:

MATCH SIMPLE allows any of the foreign key columns to be null; if any of them are null, the row is not required to have a match in the referenced table.

A (somewhat less reliable and more expensive) alternative would be a trigger on INSERT / UPDATE in address and trigger(s) on INSERT / UPDATE / DELETE in verified_name.

Related Question