“I would like my customers to be able to browse my catalog of books and place orders over the internet. Currently, I take orders over the phone. I have mostly corporate customers who call me and give me the ISBN number of a book and quantity; they often pay by credit card. I then prepare a shipment that contains the books they ordered. If I don’t have enough copies in stock, I order additional copies from the publisher and delay the shipment until the new copies arrive; I want to ship a customer’s entire order together. My catalog includes all the books I sell. For each book, the catalog contains its ISBN number, title, author, purchase price, sales price, and the year the book was published. Most of my customers are regulars, and I have records with their names and addresses. New customers have to call me first and establish an account before they can use my website.
On my new website, customers should first identify themselves by their unique customer identification number. Then they should be able to browse and to place orders online. ”
The ISBN code identifies each book uniquely and is hence the primary key of the book entity set. Same is the case with the C-id attribute of the customers entity set. Card_num has been placed as an attribute of the order because a customer may have more than one card number. There are no directed lines as one customer can order any number of books and one book may be ordered by any number of customers. Since qty, order_date, ship_date are specific to the order, they are listed as attributes of the relationship.