Held and held are optionally inclusive entity-relationship modeling

Data Modeling and Database Design

Chapter 3:
Entity-Relationship Modeling

Chapter 3 – Entity-Relationship Modeling	2

The ER Model

	3

The ER Model (continued)

	4

Chapter 3 – Entity-Relationship Modeling	5

E1	m	R	n	E2
E1		R		E2	Chen's Notation
E1		R		E2

E1 Optionally Related To E2

E1	R	E2	(Partial Participation of E1 in R)


	Noninclusive Arc	Exclusive Arc

Figure 3.2 Summary of Presentation Layer ER diagram notation (continued)

• Heuristic (Intuitive), iterative process• Recursive incremental refinement
• Target Audience: End-user community• Technology-independent

Chapter 3 – Entity-Relationship Modeling	7

• Review leftover data elements and investigate the possibility that some of them serve as links among the entity types previously identified

• Designate these links as relationship types

	8

• Be sensitive throughout the process to the
identification of relationships among the various entity types

Chapter 3 – Entity-Relationship Modeling	9

– Dependent
– Credit Union Account
– Hobby

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	- Company x - Manager x- Supervisor x

– Dependent
– Credit Union Account
– Hobby

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• Version 0 of ER diagram results: a punctuated equilibrium

• Go back and read the story (strike out story lines already captured if need be) and refine entity types and relationship types of the ER diagram based on additional facts discerned

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Box 1	(continued)

Bearcat Incorporated is a manufacturing company that has several plants in the northeastern part of the United States. These plants are responsible for leading different projects that the company might undertake, depending on a plants’ function. A certain plant might even be associated with several projects but a project is always under the control of just one plant.Some plants do not undertake any projects at all. If a plant is closed down, the projects undertaken by that plant cannot be canceled. The project assignments from a closed plant must be temporarily removed in order to allow the project to be transferred to another plant.

PROJECT

Note: The relationship shown models the italicized text

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Box 2

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Development of Presentation Layer ER Model

Box 3

Note: The relationship shown models the italicized text

	15

Chapter 3 – Entity-Relationship Modeling	16

Box 4	1		20
Box 4	1	Supervised_by	20

1 n

Held_by_E

BCU_ACCOUNT

Chapter 3 – Entity-Relationship Modeling 17

Dependent_of

1	Supervised_by

Held_by_E	1
m	1

Held_by_D

	18

Business rule: All BCU accounts are joint-accounts between an employee and a dependent

Box 6

1	Supervised_by		n

DEPENDENT

BCU_ACCOUNT

EMPLOYEE

1	Supervised_by

Held_by_E 1

Note: The noninclusive arc indicates that the relationship types Held_by_E and Held_by_D are mutually noninclusive; i.e., Held_by_E and Held_by_D are optionally inclusive.

Chapter 3 – Entity-Relationship Modeling	20