Please answer correctly I really need help! Develop an EER model for the followi
ID: 3884907 • Letter: P
Question
Please answer correctly I really need help!
Develop an EER model for the following situation. You need to make your own (reasonable) assumptions regarding the relationship cardinality if it is not explicitly indicated. Show your assumptions in your solution. You need to indicate which attribute you choose for the identifier of each entity type. You may create an identifier attribute if needed.
A technology company provides offerings to its customers. Offerings are of two separate types: products and services. Offerings are identified by an offering ID and an attribute of description. In addition, products are described by product name, standard price, and date of first release; services are described by name of the company’s unit responsible for the service, conditions of service, and the service type. There are repair, maintenance, and other types of services. A repair service has a cost and is the repair of some product offered by the company; a maintenance service has an hourly rate. The company wants to keep track of repair services on products with repair date and a short description of the issue. Some products may never require repair, but there are many potential repair services for a product. A customer may purchase an offering, and the company needs to keep track of when the offering was purchased and the contact person for that offering with the customer. Unfortunately, not all offerings are purchased. Customers are identified by customer ID and have descriptive data of name, address, and phone number. When a service is performed, that service is billed to the customer. When a customer is billed for a service (some customers may never require a service), the company wants to keep track of the service date, the due day of the bill, and the amount due.
Explanation / Answer
An ERD is a graphical description of the data for a particular database = a graphical data model. It represents the data at a high level of abstraction.
The term ‘database’ above usually implies a computer-based database; however, given the complexity of the ERD it might well be realised (‘implemented’) in the form of some type of paper based clerical system.
‘High level of abstraction’ means that it is not necessary to show details of the various fields or indexes, just the bare bones.
Opposite is a very simple ERD that shows three entity types and two relationships. On the following pages we will consider each of these elements separately.
An entity is a ‘thing’ or ‘concept’ or ‘object’. Well, most of the time”.
An entity [type] is not a single ‘thing’ but rather a representation of like or similar things that share characteristics (properties). For example, King Lear and Hamlet are both plays and have properties such as name, author, cast of actors and lines of verse. The entity [type] describing these might be PLAY, with King Lear and Hamlet as examples of instances or occurrences of PLAY [i.e. each is an entity [instance]].
While the above examples probably list a selection of the important entity types for each of the contexts, there is no absolute way of being able to say they are correct. Rumbaugh et al (1991, p21) makes this important point when discussing object identification; which for the moment you can assume is the same as an entity instance:
“We define an object as a concept, abstraction, or thing with crisp boundaries and meaning for the problem at hand. Objects serve two purposes: they promote understanding of the real world and provide a practical basis for computer implementation. Decomposition of a problem into objects depends on judgment and the nature of the problem. There is no one correct presentation.”
This is clear from considering the last example concerning car component details. Assume that another garage owner who was less concerned with collecting detailed information for each component only wanted one item of information recorded for each component. In this instance we would probably have CAR as the entity type and each of the entity types listed above relegated to attributes of the CAR entity type. There is no definitive correct answer; we only know by talking to the actual garage owner. Entities are largely defined by the context.