Chapter 2, Problem 2.23PAE

A cellular operator needs a database to keep track of its customers, their subscription plans, and the handsets (mobile phones) that they are using. The E-R diagram in Figure 2-24 illustrates the key entities of interest to the operator and the relationships between them. Based on the figure, answer the following questions and explain the rationale for your response. For each question, identify the element(s) in the E-R diagram that you used to determine your answer.

1. 
   Can a customer have an unlimited number of plans?
2. Can a customer exist without a plan?
3. Is it possible to create a plan without knowing who the customer is?
4. Does the operator want to limit the types of handsets that can be linked to a specific plan type?
5. Is it possible to maintain data regarding a handset without connecting it to a plan?
6. Can a handset be associated with multiple plans?
7. Assume a handset type exists that can utilize multiple operating systems. Could this situation be accommodated within the model included in Figure 2-24?
8. Is the company able to track a manufacturer without maintaining information about its handsets?
9. Can the same operating system be used on multiple handset types?
10. There are two relationships between Customer and Plan. Explain how they differ.
11. Characterize the degree and the cardinalities of the relationship that connects Customer to itself. Explain its meaning.
12. Is it possible to link a handset to a specific customer in a plan with multiple customers?
13. Can the company track a handset without identifying its operating system?