Describe the library model. There are 3 entities: "Author", "Book", "Reader". Physically, there is only one book and it can only be with one reader. Create tables for the library considering this.

To describe a library model with entities "Author", "Book", and "Reader" and the condition that each book is unique and can only be with one reader, the following tables can be proposed:

1. Author

• id (PK)
• name
• other fields (biography, date of birth, etc.)

2. Book

• id (PK)
• title
• author_id (FK to Author.id)
• reader_id (FK to Reader.id, nullable)

3. Reader

• id (PK)
• name
• other fields (contact details, etc.)

Explanation:

• Each book is linked to one author via author_id.
• The reader_id field in the books table indicates which reader currently has the book. If the book is not issued, this field is NULL.
• Since a book can only be with one reader, the "book — reader" relationship is implemented through a foreign key in the books table.

Thus, the model reflects the uniqueness of the book and its belonging to one reader.

Example SQL schema:

CREATE TABLE Author (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

CREATE TABLE Reader (
  id SERIAL PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);

CREATE TABLE Book (
  id SERIAL PRIMARY KEY,
  title VARCHAR(255) NOT NULL,
  author_id INTEGER NOT NULL REFERENCES Author(id),
  reader_id INTEGER REFERENCES Reader(id)
);