Information Resources
Management
February 6, 2001
Agenda
Administrivia
 Advanced E-R Modeling

Administrivia
Homework #2
 Homework #3

Difficulties with Previous E-R
Modeling
employee’s billing rate may vary by
project
 attribute of the relationship between
property and owner is percent owned
 different types of bank accounts
 customer must have at least one
account of any type
 tracking transactions

Also, Cannot Handle
Situations Like

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different kinds of property for sale
 single family, duplex, apartment building,
commercial, industrial
different kinds of projects
 internal, DoD, joint venture, other govt,
other
employee assigned to a project has a specific
location just for that project
billing rate for an employee can vary by both
project and location
Solutions
Associative Entities
 Ternary (n-ary) relationships
 Generalization/Specialization
 Time Dependent Data (weak entities)

Associative Entities
A relationship cannot have attributes
 What happens when one or more
attributes exist for a relationship?
 Employee – Project – Billing Rate

Associative Entities
 An
associative entity is an entity type
that associates the instances of more
or more entity types and contains
attributes that are specific to the
relationship between those entity
instances.
 NOUN
Converting a relationship to an
associative entity

Example
 Employee’s billing rate can vary by
project (employee to project, many-tomany)
Employee
Billing
Rate
Project
Example:
 Attribute
of the relationship between
property and owner is percent owned
Owner
%
Owned
Property
Example:

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Students take one or more courses
A course could be offered before any students
register for it
Students are awarded mid-term and final grades for
each course for which they are registered
Attributes for students include student ID, name,
department
Attributes for course include course number, name,
department
Draw the E-R diagram
Ternary Relationship

simultaneous relationship among one
instance from each of exactly three
entity sets
 An associative entity is not required
but is typical.
Ternary Relationships –
Cardinality

“one” – one instance of an entity is
associated with each unique pair of the
other two entities.

“many” – more than one instance of an
entity is associated with each unique
pair from each of the other two entities.
Ternary Relationships Cardinality

For the entity in question,
 For one instance of the entity,
 How many unique pairs of the other
entities can there be?
 One
 More than one (many)
Example
(one-to-one-to-one):
Employee is assigned a phone number
for a project. A phone number is used
only for that employee and project.
Example (one-to-one-to-one):
Employee
Has
Project
Phone
Number
Note: An employee could still be assigned to multiple projects but
would have a unique phone number for each project assignment.
Example
(one-to-one-to-many):
Employee assigned to a project works
at one location for that project but can
work at different locations for different
projects. At a location an employee
only works on one project, but there
can be many employees working on
that same project.
Example
(one-to-one-to-many):
Employee
Has
Location
Project
Example
(one-to-many-to-many):
Employee on a project has one
manager. Manager can manage
several projects. Each project has
one manager. Manager can manage
the same employee on different
projects.
Example
(one-to-many-to-many):
Employee
Has
Manager
Project
Example
(many-to-many-to-many):
Employees use many skills on many
projects and each project has many
employees with varying skills.
Example
(many-to-many-to-many):
Employee
Has
Skill
Project
Ternary Relationships
 Why
not just make the ternary
relationship an associative entity?
Employee
Location
Project
Good idea if possible.
However,
“Business Rules”
 Need for separate (strong) entity
 What if location also related to other
entities?
 No longer valid as associative entity

If you can safely get rid of ternary (or n-ary) relationships, DO IT.
However, don’t trade correctness for ease of implementation.
Example:

Employee assigned to a project works
at one location and is only assigned to
one project. However, projects can be
at more than one location and have
multiple employees working on them at
each location. Multiple projects can be
at the same location.
Draw the E-R Diagram.
Example:

Now, suppose employees have a billing
rate that varies with both project and
location.
Update your E-R Diagram.
Generalization/
Specialization
 Generalization:
Defining a general
entity type from a set of more
specialized ones (bottom-up).
 Specialization:
Defining one or more
specialized entities from a more
general one based on distinguishing
characteristics
(top-down).
Examples:
 Generalization:
Bank account
generalized from checking, savings,
and loan.
 Specialization:
Property for sale
divided into single family, duplex,
apartment, commercial, or industrial.
Why use generalization or
specialization?

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clarity
more fully describe situation
conversion to OO (inheritance)
different relationships only apply to specific subtypes
 monthly charges only made to checking accounts

different relationships only apply to the supertype
 customer must have an account with the bank

different (additional) attributes for the subtypes
Generalization/
Specialization
Applies to entities only
 Types:
 disjoint – can be only one subtype
 overlapping – can be more than one
subtype


Completeness Constraint – subtypes
fully inclusive of supertype
Generalization/
Specialization

Disjoint – student can be
undergraduate, masters, or doctoral

Overlapping – major can be history,
philosophy, or mathematics
Completeness Constraint

Disjoint:
 Completeness – student must be one
and only one of the three
 None – “at large” student is simply a
student and the additional
specialization isn’t needed
Completeness Constraint

Overlapping:
 Completeness – major must be at
least one of the three listed
 None - other majors not individually
broken out but possible
Drawing It – Disjoint
d
 - subset and direction indicator
Drawing It – Overlapping
O
Drawing It – Completeness
Constraint
Double line
d
O
Examples
Student
d
underg
masters
doctor
Examples
Major
O
history
philosp
math
Time Dependent Data
keep history over time
 need to track changing attribute and
know its past values


“timestamp”
 year, month, day, etc…
 as precise as nanoseconds
Two Approaches
multivalued attribute
 additional entity (weak)

Example

A company sells products whose price
can change at most once a day. Need
to be able to track the price history.
Multivalued Attribute
Product ID
Product
Description
Price
Price History
Product Type
Effective Date
Additional Entity
Product
Has
Why is price history a weak entity??
Price
History
Warnings for Time Dependent
Data

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Is it necessary?
How long must it be kept?
Will it get “cleaned up”? How? When?
Do other entities have relationships to the
history information?
 What
happens to sales by product
type if the product type is changed?

Exactly how much historical information
needs to be kept? What attributes have
historical values?
The Steps in E-R Modeling
classify entities and attributes
 identify generalization hierarchies
 define relationships


All three simultaneous and iterative
Exercise 1
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A real estate firm lists property for sale. The following describes this
organization:
The firm has a number of sales offices in several states
Each sales office is assigned one or more employees. An employee
must be assigned to only one sales office.
For each sales office, there is always one employee assigned to
manage that office. An employee may manage only the sales office to
which she is assigned.
The firm lists property for sale.
Each unit or property must be listed with one (and only one) of the
sales offices. A sales office may have any number of properties listed,
or may have no properties listed.
Each unit of property has one or more owners. An owner may own
more than one unit of property. An attribute of the relationship between
property and owner is percent owned.
Exercise 1 - New
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Property listed must be at least one of the following types:
single family, apartment, commercial, industrial.
Single family property can be a house, duplex, condo, or
townhouse. More than one is possible (ie, condo and
townhouse)
A history of previous listings should be kept along with the sales
prices and selling employees.
In a sales office, each employee has security access to one or
more PCs. Managers have access to all PCs in their office as
well as other offices. Different employees can have different
levels of access. This access needs to be tracked.
Zoning information is kept for commercial and industrial
properties only.
Gifts of varying values (based on the purchase price) are given
to the purchasers of single family houses or townhouses only.
Exercise 2

Construct an E-R diagram for a bank
database that shows the basic relationships
among customers, checking accounts,
savings accounts, loans, loan officers, and
the bank branches where various accounts
and loans are taken out. You also want to
keep track of transactions on accounts and
loans, and maintain the current balance in
each account and the balance of the loan.
Exercise 2 - New

CDs (certificates of deposit) are also offered. A CD must be one
of four types – small, medium, large, and jumbo. The interest
rate and minimum term vary with the type of CD and customer.
“Special” customers (this is indicated in the customer
information) receive higher interest rates or shorter terms.
Customers who open a large or jumbo CD get to choose a free
gift from a list of available ones. The gifts for each type of CD
are different.

A loan officer may work at more than one branch. When an
officer is assigned to multiple branches, she has a unique phone
number at each branch. All other bank employees each have a
unique phone number.