Postgresql multiple tables with same foreign key unique constraint - sql

Question

I have following tables on PostgreSQL 9.4
CREATE TABLE "user" (
id SERIAL PRIMARY KEY,
email CHARACTER VARYING NOT NULL,
password CHARACTER VARYING NOT NULL
);
CREATE TABLE "dealer" (
id SERIAL PRIMARY KEY,
user_id INTEGER NOT NULL REFERENCES "user" (id) ON DELETE RESTRICT
);
CREATE TABLE "affiliate" (
id SERIAL PRIMARY KEY,
user_id INTEGER NOT NULL REFERENCES "user" (id) ON DELETE RESTRICT
);
Is it possible to force user_id value to be unique across tables dealer and affiliate?

Answer 1

There are different setups to use for inheritance in SQL and for this you could just use an integer column type in the table user that marks the type of the user and would reference to table user_type (id,name) that would have the values 1,dealer and 2,affiliate:
CREATE TABLE user_type (
id INTEGER PRIMARY KEY, --could be SERIAL
name text
);
INSERT INTO user_type VALUES (1,'dealer'), (2, 'affiliate');
CREATE TABLE "user" (
id SERIAL PRIMARY KEY,
email CHARACTER VARYING NOT NULL,
password CHARACTER VARYING NOT NULL,
user_type INTEGER REFERENCES user_type NOT NULL,
UNIQUE(id,user_type)
);
This in itself wouldn't force uniqueness across tables so after implementing this you would have the following options:
Drop the tables dealer and affiliate - you won't need them if you rely on the type field to see which one the user is.
If you have to keep those inherited tables you can:
Use triggers - these triggers check the uniqueness and would be actived on INSERT or UPDATE
Another (a bit clumsy) solution: add user_type field to both subtables like this:
CREATE TABLE "dealer" (
id SERIAL PRIMARY KEY,
user_id INTEGER NOT NULL,
user_type INTEGER NOT NULL DEFAULT 1 check (user_type = 1),
FOREIGN KEY (user_id,user_type) REFERENCES "user"(id,user_type) ON DELETE RESTRICT
);
CREATE TABLE "affiliate" (
id SERIAL PRIMARY KEY,
user_id INTEGER NOT NULL,
user_type INTEGER NOT NULL DEFAULT 2 check (user_type = 2),
FOREIGN KEY (user_id,user_type) REFERENCES "user"(id,user_type) ON DELETE RESTRICT
);
The checks and foreign keys together make sure you cannot have both types of user in the main table. Note that user_id might be used as the PRIMARY KEY in the subtables too. Currently a row in user might have several dealer rows linked to it so at least you might want to set user_id foreign keys in subtables as UNIQUE.