Database Reference
In-Depth Information
A weak entity type depends on another entity type for its existence.
The notion of generalization and specialization in the E-R model is similar to
the same concept in the object-based data model.
REVIEW QUESTIONS
1. “The E-R data modeling technique focuses on entities and relationships as
fundamental model components.” Explain.
2. Define the terms entity, entity type, and entity set.
3. What are physical and conceptual entities? Give two examples for each.
4. Describe attributes and value domains with three examples.
5. What is a composite attribute? Can a composite attribute itself be part of
another attribute? If so, can you think of an example?
6. Describe the considerations for choosing the primary key for an entity type.
7. What is the degree of a relationship? Give two examples.
8. Give an example of a situation that needs modeling time-dependent
components.
9. What is an identifying relationship? Explain briefly with an example.
10. What is a gerund? Why would you use a gerund in a data model? Give an
example.
EXERCISES
1. Match the columns:
1. domain A. three-way relationship
2. composite attribute B. one or more simple attributes
3. relationship C. time-dependent
4. ternary relationship D. not absolutely essential in a model
5. minimum cardinality E. no independent primary key
6. historical entity type F. unary relationship
7. weak entity type G. has distinct notation in E-R model
8. recursive relationship H. association between entities
9. derived attribute I. allowable values for attribute
10. primary key J. indicates optionality of relationship
2. As a data modeler for a car dealership, identity the major entity types. List
the attributes and primary keys for two of the entity types.
3. The health care coverage for employees in your company extends to employee
dependents. Create a partial E-R data model to represent the health insur-
ance coverage. (Hint: use identifying and nonidentifying relationships.)
4. Consider the relationship between the entity types WORKER and
BUILDING. Assume variations in business rules with optional and manda-
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