Chapter 5

 1.  What is an entity type?
Ans: An entity type is a collection of things of interest (entities) in an application. Entity types can represent physical objects such as books, people, and places, as well as events such as payments.

 2.  What is an attribute?
Ans: An attribute is a property of an entity type or relationship. Each attribute has a data type that defines the kind of values and permissible operations on the attribute.

 3.  What is a relationship?
Ans: A relationship is a named association among entity types. A relationship represents a two-way or bi-directional association among entities. Most relationships involve two distinct entity types.

4.   What is the natural language correspondence for entity types and relationships?
Ans: Entity types can correspond to nouns and relationships to verbs or prepositional phrases connecting nouns. If practical, it is a good idea to use active rather than passive verbs for relationships. You should use the natural language correspondence as a guide rather than strict rule. For large ERDs, you will not always find a good natural language correspondence for all parts of the diagrams.

 5.  What is the difference between an ERD and an instance diagram?
Ans: An ERD has the relationship name on the line connecting the entity types involved in the relationship. An instance diagram doesn’t have the relationship name on the connecting line. Instead, an instance diagram shows a set of entities and connection lines between the members in the entities to represent the relationship.

 6.  What symbols are the ERD counterparts of foreign keys in the Relational Model?
Ans: Named relationships with cardinality symbols are the counterparts of foreign keys.

 7.  What cardinalities indicate functional, optional, and mandatory relationships?
Ans: A maximum cardinality of one indicates a functional relationship. A minimum cardinality of zero indicates an optional relationship. A minimum cardinality of one or more indicates a mandatory relationship.

 8.  When is it important to convert an M-N relationship into 1-M relationships?
Ans: When an M-N relationship must be related to other entity types in another relationship, you should convert an M-N relationship into two 1-M relationships.

 9.  How can an instance diagram help to determine whether a self-referencing relationship is a 1-M or an M-N relationship?
Ans: Since an instance diagram shows each member in a set of entities, the relationship among members is clearly depicted. To determine whether a self-referencing relationship is 1-M or M-N, you should look to see if entities can be connected to multiple entities in both directions of the relationship.

 10. When should an ERD contain weak entities?
Ans: An ERD contains weak entity types when entities are closely associated with other entities. Identification dependency often occurs when entities are physically located inside another entity such as rooms inside a building.

 11. What is the difference between an existence-dependent and a weak entity type?
Ans: A weak entity is a specialized kind of existence-dependent entity. Like an existent-dependent entity, a weak entity has a mandatory relationship. In addition to the existence dependency, weak entities borrow part or their entire primary key.

12. Why is classification important in business?
Ans: Classification helps people better understand the business environment by understanding the similarities and differences among business entities.  Classification can be applied to investments, employees, customers, loans, parts, and so on.

13. What is inheritance in generalization hierarchies?
Ans: Inheritance is a data modeling feature that supports sharing of attributes between a supertype and a subtype.

14. What is the purpose of disjointness and completeness constraints for a generalization hierarchy?
Ans: Disjointness and completeness constraints depict relationships among the sets of entities in the subtypes of a generalization hierarchy. Disjointness and completeness constraints cannot be expressed using the cardinality constraints of an ERD.

15. What symbols are used for cardinality in the Crow’s Foot notation?
Ans: The Crow’s Foot symbol (two angled lines and one straight line) denotes the many (zero or more) related entities. The circle symbol means a cardinality of zero, while a line perpendicular to the relationship line means a cardinality of one.

16. What are the two components of identification dependency?
Ans: Identification dependency involves a weak entity and one or more identifying relationships.

17. How are M-way relationships represented in the Crow’s Foot notation?
Ans: The Crow’s Foot notation cannot directly represent M-way relationships because the Crow’s Foot notation only supports binary relationships. You use an associative entity type and a collection of binary relationships to represent an M-way relationship.

18. What is an associative entity type?
Ans: An associative entity type is a weak entity that replaces an M-way relationship. An associative entity type depends on two or more entity types for its primary key.

19. What is the equivalence between a M-N relationship and 1-M relationships?
Ans: A M-N relationship can be replaced by an associative entity type and two identifying 1-M relationships.

20. What is does it mean to say that part of a primary key is borrowed?
Ans: Identification dependency allows an entity type (weak entity) to be identified through one or more entity types. The weak entity borrows all or part of its primary key from other entity types.

21. What is the purpose of the diagram rules?
Ans: The diagram rules ensure that an ERD does not contain obvious notational errors. The diagram rules are similar to syntax rules for a computer language.

22. What are the limitations of the diagram rules?
Ans: The diagram rules do not ensure that you have considered multiple alternatives, correctly represented user requirements, and properly documented your design.

23. What consistency rules are commonly violated by novice data modelers?
Ans: Consistency rules 6 through 9 are common sources of errors for novice data modelers.

24. Why do novice data modelers violate the identification dependency rules (consistency rules 6 through 8)?
Ans: Identification dependency involves a weak entity and identifying relationships so there are more sources of errors than other parts of the Crow’s Foot notation. Each identifying relationship also requires a minimum and maximum cardinality of 1 that is also overlooked by novice data modelers.

25. Why do novice data modelers violate consistency rule 9 about redundant foreign keys?
Ans: Novice data modelers violate consistency rule 9 because of confusion between an ERD and the relational data model.

26. Why should a CASE tool support diagram rules?
Ans: CASE tools should support diagram rules to improve the productivity of data modelers, especially novice data modelers.

27. How does the ER Assistant support consistency rules 4 and 5?
Ans: The ER Assistant supports consistency rules 4 and 5 through its diagramming tools. Relationships must be connected to two entity types (not necessarily distinct) prohibiting violations of consistency rules 4 and 5.

28. How does the ER Assistant support all rules except consistency rules 4 and 5?
Ans: For the other completeness and consistency rules, the ER Assistant provides the Check Diagram button that generates a report of rule violations.

29. Why does the ER Assistant not require resolution of all diagram errors found in an ERD?
Ans: Because the Check Diagram button may be used when an ERD is not complete, the ER Assistant does not require fixing rule violations found in an ERD. In addition, some diagram rules such as completeness rules 4 and 5 may be violated in certain situations.

30. How does the ER Assistant implement consistency rule 9 about redundant foreign keys?
Ans: For consistency rule 9, the ER Assistant uses a simple implementation to determine if an ERD contains a redundant foreign key. The ER Assistant checks the child entity type (entity type on the many side of the relationship) for an attribute with the same name and data type as the primary key in the parent entity type (entity type on the one side of the relationship). If the ER Assistant finds an attribute with the same name and data type, a violation is listed in the Check Diagram Report.

31. List some symbol differences in ERD notation that you may experience in your career.
Ans: Students' responses may vary. The Chen notation uses a diamond to represent the relationship, while the Crow’s foot notation has the name on the line. For minimum and maximum cardinality, the Chen notation presents 0, 1, and N in parentheses. Some notations reverse the position of the cardinalities. Some notations display names for both directions of a relationship. Other symbol variations are visual representations for certain kinds of entity types.  In some notations, weak entities and M-N relationships have special representations.  Weak entities are sometimes enclosed in double rectangles.  Identifying relationships are sometimes enclosed in double diamonds.  M-N relationships with attributes are sometimes shown as a rectangle with a diamond inside denoting the dual qualities (both relationship and entity type).

32. List some diagram rule differences in ERD notation that you may experience in your career.
Ans: Students' responses may vary. Some notations do not support M-way relationships, M-N relationships, relationships with attributes, and self-referencing (unary) relationships. Some notations support relationships to be connected to other relationships and attributes to have more than one value (multi-valued attributes).

33. What is the Unified Modeling Language (UML)?
Ans: The Unified Modeling Language is the standard notation for object-oriented modeling. The UML contains class diagrams, interface diagrams, and interaction diagrams to support object-oriented modeling. The class diagram notation provides an alternative to the ERD notations presented in this chapter.

34. What are the modeling elements in a UML class diagram?
Ans: Class diagrams can contain classes (collections of objects), associations (binary relationships) among classes, and object features (attributes and operations). In addition, class diagrams can include generalization hierarchies and composition relationships.

35. What kinds of business rules can be defined through elements of an ERD?
Ans: An ERD can formally represent entity identification through primary keys, connections among business entities through relationships, the number of related entities through cardinalities, and inclusion among entity sets through generalization hierarchies.

36. What kinds of business rules are defined through informal documentation in the absence of a rules language for an ERD?
Ans: In the absence of a formal rules language, informal documentation can be used to define alternative entity identification, attribute comparison constraints, and data collection completeness constraints.