Multiple yet mutually exclusive foreign keys - is this the way to go?

Question

I have three tables: Users, Companies and Websites. Users and companies have websites, and thus each user record has a foreign key into the Websites table. Also, each company record has a foreign key into the Websites table.

Now I want to include foreign keys in the Websites table back into their respective "parent" records. How do I do that? Should I have two foreign keys in each website record, with one of them always NULL? Or is there another way to go?

Answer 1

Why do you need a foreign key from website to user/company at all? The principle of not duplicating data would suggest it might be better to scan the user/company tables for a matching website id. If you really need to you could always store a flag in the website table that denotes whether a given website record is for a user or a company, and then scan the appropriate table.

Answer 2

you don’t need a parent column, you can lookup the parents with a simple select (or join the tables) on the users and companies table. if you want to know if this is a user or a company website i suggest using a boolean column in your websites table.

Answer 3

First of all, do you really need this bi-directional link? It is a good practice to avoid it unless absolutely needed.

I understand it that you wish to know whether the site belongs to a user or to a company. You can achieve that by having a simple boolean field in the Website table - [BelongsToUser]. If true, then you look up a user, if false - you look up a company.

Answer 4

If we look into the model here, we will see the following:

1. A user is related to exactly one website
2. A company is related to exactly one website
3. A website is related to exactly one user or company

The third relation implies existence of a "user or company" entity whose PRIMARY KEY should be stored somewhere.

To store it you need to create a table that would store a PRIMARY KEY of a website owner entity. This table can also store attributes common for a user and a website.

Since it's a one-to-one relation, website attributes can be stored in this table too.

The attributes not shared by users and companies should be stored in the separate table.

To force the correct relationships, you need to make the PRIMARY KEY of the website composite with owner type as a part of it, and force the correct type in the child tables with a CHECK constraint:

CREATE TABLE website_owner (
    type INT NOT NULL,
    id INT NOT NULL,
    website_attributes,
    common_attributes,
    CHECK (type IN (1, 2)) -- 1 for user, 2 for company
    PRIMARY KEY (type, id)
)

CREATE TABLE user (
    type INT NOT NULL,
    id INT NOT NULL PRIMARY KEY,
    user_attributes,
    CHECK (type = 1),
    FOREIGN KEY (type, id) REFERENCES website_owner
)

CREATE TABLE company (
    type INT NOT NULL,
    id INT NOT NULL PRIMARY KEY,
    company_attributes,
    CHECK (type = 2),
    FOREIGN KEY (type, id) REFERENCES website_owner
)