Describe and discuss the main elements of the Entity-Relationship (E-R) model. Include an example.

The E-R model is basically made up of entities, attributes and relationships. Entities represent the things people want to keep track of, such as customers and products. Each "thing" is presented by an entity or entity class–in this case CUSTOMER and PRODUCT. Individual members of each entity class are called entity instances–for example CUSTOMER John Smith and PRODUCT Canned Tomato Soup. Each entity is described by a set of attributes. For example, a CUSTOMER will have a CustomerNumber, a LastName and a FirstName, while a PRODUCT will have a ProductNumber and a Description. These, of course, have specific values for each entity instance. Finally, the entities are connected by relationships. There are relationship classes between entity classes and relationship instances between entity instances. For example, in general a CUSTOMER buys a PRODUCT, and a PRODUCT is sold to a CUSTOMER. Specifically, CUSTOMER John Smith buys a PRODUCT Canned Tomato Soup.