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Topics: Database design, Data modeling, Entity-relationship model Pages: 6 (1653 words) Published: October 29, 2013
12.8Describe how strong and weak entity types differ and provide an example of each.

A strong entity type is an entity type that is not existence-dependent on some other entity type A weak entity type is an entity type that is existence-dependent on some other entity type.


Strong entity weak entity Client is strong entity because it already has primary key and doesn’t depend on any other entity, but the reference is weak entity because depends on client entity and doesn’t have primary key. (Section 12.4).

12.11You are required to create a conceptual data model of the data requirements for a company that specializes in IT training. The Company has 30 instructors and can handle up to 100 trainees per training session. The Company offers five advanced technology courses, each of which is taught by a teaching team of two or more instructors. Each instructor is assigned to a maximum of two teaching teams or may be assigned to do research. Each trainee undertakes one advanced technology course per training session.

(a) Identify the main entity types for the company.

(b) Identify the main relationship types and specify the multiplicity for each relationship. State any assumptions you make about the data.

(c) Using your answers for (a) and (b), draw a single ER diagram to represent the data requirements for the company.

Chapter 13 Extended Entity-Relationship Modelling

13.7- Describe the two main constraints that apply to a specialization/generalization relationship. There are two constraints that may apply to a superclass/subclass relationship called participation constraints and disjoint constraints.

Participation constraint determines whether every occurrence in the superclass must participate as a member of a subclass. A participation constraint may be mandatory or optional. A superclass/subclass relationship with a mandatory participation specifies that every entity occurrence in the superclass must also be a member of a subclass. A superclass/subclass relationship with optional participation specifies that a member of a superclass need not belong to any of its subclasses.

Disjoint constraint describes the relationship between members of the subclasses and indicates whether it’s possible for a member of a superclass to be a member of one, or more than one, subclass. The disjoint constraint only applies when a superclass has more than one subclass. If the subclasses are disjoint, then an entity occurrence can be a member of only one of the subclasses. To represent a disjoint superclass/subclass relationship, an ‘Or’ is placed next to the participation constraint within the curly brackets. If subclasses of a specialization/generalization are not disjoint (called nondisjoint), then an entity occurrence may be a member of more than one subclass. The participation and disjoint constraints of specialization/generalization are distinct giving the following four categories: mandatory and nondisjoint, optional and nondisjoint, mandatory and disjoint, and optional and disjoint.

13.8- Describe and contrast the concept of aggregation and composition and provide an example of each.

Aggregation: represents a “has a” or “is part of” relationship between entity types, where one represents the “whole” and the other the “part”. The relationship represents an association between two entity types that are conceptually at the same type. Example: Has relationship which relates the branch entity to the staff entity. Branch entity is the whole when the staff entity is the part of the branch. See figure 13.9 for better example.

Composition: a specific form of aggregation that represents an association between entities, where there is a strong ownership and coincidental lifetime between the “whole” and the “part”. Example: the...
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