How to normalize relationships between tables with references to a parent table

Question

Disclaimer that I'm not very versed in relational database design and normalization, but this is something that I've encountered and I'm not sure if this is a normalized design. Let's say there's a parent "company" table

companies

id	name
1	company 1
2	company 2

and there are Books and Authors that are created under a company

books

id	company_id	name
1	1	Red Riding Hood
2	2	Goosebumps

authors

id	company_id	name
1	1	Joe
2	2	Bob

Now if we want to assign books to authors or vice versa:

books_authors

book_id	author_id
1	1
2	2

but we'll also be permitted to connect books and authors that are not under the same company:

book_id	author_id
1	2

Is there a better design than this that would prevent books and authors of different companies from being associated with each other? Do I just have to add a check/constraint to prevent that, or is there some other way to tackle this?

Answer 1

As you have it set up, the company owns the book and the author. This doesn't match reality; author(s) write books, companies publish books, and a book can have many editions published by many companies .

Formally:

• A book has many authors.
• A book has many editions.
• An edition has a company (the publisher).
• A book has many companies through their editions.

And from that we can derive...

• An author has many books.
• An author has many editions of their books.
• An author has many companies through their book's editions.
• A company has many editions.
• A company has many books through their editions.
• A company has many authors through their editions' books.

-- These can be publishers.
create table companies (
  id bigserial primary key,
  name text not null
);

-- These can be authors.
create table people (
  id bigserial primary key,
  name text not null
);

create table books (
  id bigserial primary key,
  title text not null
);

-- This can also include their role in the authoring.
-- For example: illustrator, editor
create table book_authors (
  book_id bigint not null references books,
  person_id bigint not null references people,

  -- Can't author the same book twice.
  unique(book_id, person_id)
);

-- This can also include information about the publishing such
-- as its year and language.
create table book_editions (
  book_id bigint not null references books,
  company_id bigint not null references companies,
  edition text not null,

  -- The same company can't publish the same edition twice.
  unique(book_id, company_id, edition)
);

This is a generic, flexible book/author/publisher schema. However, the schema should be primarily defined by how you intend to use it.