You 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", "Reader" and the condition that each book physically exists as a single copy and can only be with one reader at a time, the following tables can be used:

1. Author

• id (PK)
• name

2. Book

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

3. Reader

• id (PK)
• name

Explanation:

• Each book is linked to one author via author_id.
• The reader_id field in the book table indicates which reader currently has the book. If the book is not issued, this field will be NULL.

This ensures that each book exists as a single copy and can only be with one reader at a time.