The database keeps information about users, products, sales and bids. Each user is uniquely identified by an email address, and also has name, password, city, and gender. A product catalog maintains information about (frequently sold) products. Each product in the catalog is uniquely identified by a pid, and has a description (e.g. "Nikon F100"). Products can have reviews. Each product review is written by a user and has a rating (a number from 1 to 5), a review text, and a date when the review is written. Users can endorse product reviews (written by others). Users cannot endorse the same product review more than once. Users can also review other users and each such review has a rating (a number from 1 to 5), a review text, and a date when the review is written. The database keeps all review details. Each sale has a sale id, description, reserved price, condition, and date and time when the sale ends. Each sale is listed by one user, and may be associated to a product in the product catalog. A sale may also include a listing of the items that are included in the sale (e.g. "Nikon F100 body", "Nikon 50mm, f/1.4 lens", "Kodak 35mm film"). Each of those items may also be associated with a product in the product catalog. Users can bid on sales; each bid has a dollar amount and a date/time when the bid is placed. A user can bid multiple times on the same sale