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Topics: Data modeling / Pages: 7 (1653 words) / Published: Oct 30th, 2013

12.8 Describe 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).

12.11 You 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

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