Chapter 4
Entity Relationship (E-R) Modeling
Developing an E-R Diagram
Database design is an iterative rather than a linear or sequential process.
Begin with a general narrative of the organization's operations and procedures.
Identify entities and relations
The basic E-R model is graphically depicted and presented for review.
The process is repeated until end users and designers agree that the E-R diagram is a fair
representation of the organization's activities and functions.
Developing an E-R Diagram: Tiny College Database (1)
Tiny College (TC)
is divided into several SCHOOLs
Each school is administered by a one DEAN
Each dean administers one school.
A 1:1 relationship exists between DEAN and SCHOOL.
Administrators and Professors
Each DEAN is an ADMINISTRATOR
DEANs, however, are also PROFESSORs rank and may teach a class
Administrators and professors are also Employees.
Figure 4.38
We may wish to capture the ADMINISTRATOR vs PROFESSOR relationship with a Supertype Subtype
hierarchy.
, Figure 4.39
We might also simply treat DEANs as PROFESSORs.
Note that since not all employees are professors PROFESSOR is optional to EMPLOYEE.
Developing an E-R Diagram: Tiny College Database (2)
Each SCHOOL is composed of several DEPARTMENTs.
The smallest number of DEPARTMENTs in a school is one, and the largest number of
departments is indeterminate (N).
Each DEPARTMENT belongs to only a single SCHOOL.
Entity Relationship (E-R) Modeling
Developing an E-R Diagram
Database design is an iterative rather than a linear or sequential process.
Begin with a general narrative of the organization's operations and procedures.
Identify entities and relations
The basic E-R model is graphically depicted and presented for review.
The process is repeated until end users and designers agree that the E-R diagram is a fair
representation of the organization's activities and functions.
Developing an E-R Diagram: Tiny College Database (1)
Tiny College (TC)
is divided into several SCHOOLs
Each school is administered by a one DEAN
Each dean administers one school.
A 1:1 relationship exists between DEAN and SCHOOL.
Administrators and Professors
Each DEAN is an ADMINISTRATOR
DEANs, however, are also PROFESSORs rank and may teach a class
Administrators and professors are also Employees.
Figure 4.38
We may wish to capture the ADMINISTRATOR vs PROFESSOR relationship with a Supertype Subtype
hierarchy.
, Figure 4.39
We might also simply treat DEANs as PROFESSORs.
Note that since not all employees are professors PROFESSOR is optional to EMPLOYEE.
Developing an E-R Diagram: Tiny College Database (2)
Each SCHOOL is composed of several DEPARTMENTs.
The smallest number of DEPARTMENTs in a school is one, and the largest number of
departments is indeterminate (N).
Each DEPARTMENT belongs to only a single SCHOOL.
