A local bookstore invites you to design a database system for them based on the following requirements.

Basic Requirements

1. Books information includes book name, authors, book type, price, and units in Inventory. One book will only have on book type.

2. The store has different areas to organize different types of books. One type of book can only be located in one area.

3. An invoice system is needed to record purchasing. A customer can buy more than on book at one time.

Additional Requirements

1. Customers can search books by an author's name. However, one book may have multiple authors.

2. The book type may be two-level hierarchical. For instance, under the book type Business, we may have Human Resources, Organizational Behavior, Management, etc. The book is related to the child level type, but the area is related to the parent level type. The system should allow a customer to locate a book.

Please design a database for this bookstore to meet basic requirements and additional.

Draw the ERD, and write some descriptions to explain how the additional requirements can be addressed.