//Need to describe the library model. There are 3 entities: "Author", "Book", "Reader" Physically, there is only one book and it can only belong to one reader. You need to create tables for the library to account for this.

To describe a library model with entities "Author", "Book", and "Reader", considering that each book exists as a single copy and can be with one reader, the following tables can be proposed:

-- Authors table
CREATE TABLE authors (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

-- Books table
CREATE TABLE books (
  id SERIAL PRIMARY KEY,
  title VARCHAR(255) NOT NULL,
  author_id INT NOT NULL REFERENCES authors(id),
  reader_id INT UNIQUE REFERENCES readers(id) -- a book can be with one reader or none
);

-- Readers table
CREATE TABLE readers (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

Explanation:

• Each book is linked to one author via author_id.
• The reader_id field in the books table indicates who currently owns the book. It is unique so that one book cannot be with multiple readers.
• If the book is not issued, reader_id will be NULL.

This approach reflects the physical uniqueness of the book and its current owner.