ER Design (2)
What Can We Do?
• We Already Know How to Model
– Roles
– Cardinality constraints
– Participation
– Choice of entity sets attributes, or relationship
sets
– Binary and non-binary relationships
• However, there are more complicated
situations …
CMPT 354: Database I -- ER Design (2)
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Translating Constraints
• Translating all constraints may not be
possible – there may be instances in the
translated schema that cannot correspond
to any instance of R
– Many-to-one from A and B to C cannot be
translated if the ternary relationship is converted
into 3 binary ones
A
A
B
C
CMPT 354: Database I -- ER Design (2)
B
C
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Mapping Cardinalities Affects Design
• We can make access-date an attribute of account, instead
of a relationship attribute, if each account can have only
one customer
– Then, the relationship from account to customer is many to one, or
equivalently, customer to account is one to many
CMPT 354: Database I -- ER Design (2)
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Weak Entity Sets
• An entity set that does not have a primary
key
– The existence of a weak entity set depends on
the existence of an identifying entity set
CMPT 354: Database I -- ER Design (2)
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Weak Entity Sets (Cont.)
• A weak entity set must relate to the identifying
entity set via a total, one-to-many relationship set
from the identifying relationship set to the weak
entity set
– The identifying relationship set depicted using a double
diamond
• The discriminator (or partial key) of a weak entity
set is the set of attributes that distinguishes among
all the entities of a weak entity set
• The primary key of a weak entity set is formed by
the primary key of the strong entity set on which
the weak entity set is existence dependent, plus
the discriminator of the weak entity set
CMPT 354: Database I -- ER Design (2)
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One More Example
• In a university, a course is a strong entity
and a course_offering can be modeled as a
weak entity
– The discriminator of course_offering would be
semester (including year) and section_number
(if there is more than one section)
• How to draw the ER diagram?
CMPT 354: Database I -- ER Design (2)
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Sub-Classes and ISA Relationships
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Aggregation: Motivation
• Consider the ternary relationship works_on, which
we saw earlier. Suppose we want to record
managers for tasks performed by an employee at
a branch
CMPT 354: Database I -- ER Design (2)
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Aggregation
• Relationship sets works_on and manages
represent overlapping information
– Every manages relationship corresponds to a works_on
relationship
– However, some works_on relationships may not
correspond to any manages relationships
• So we can’t discard the works_on relationship
• Eliminate this redundancy via aggregation
– Treat relationship as an abstract entity
– Allows relationships between relationships
– Abstraction of relationship into new entity
CMPT 354: Database I -- ER Design (2)
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E-R Diagram with Aggregation
CMPT 354: Database I -- ER Design (2)
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Case Study – Student Info Sys
• Basic entities
– Students: name, address, department
– Courses: department, title, description
– Instructor: name, rank, office
name
address
student
department
department
title
description
name
rank
office
course
instructor
CMPT 354: Database I -- ER Design (2)
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Relationships
• Students take courses
• Instructors teach courses
name
address
department
department
student
title
description
name
take-course
office
course
teach
semester classroom
rank
instructor
time
semester classroom
CMPT 354: Database I -- ER Design (2)
time
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Problem: Missing Entities
• Course offering is missing
– Repeated information: semester, classroom, time
– Missing constraint: each class has one instructor
name
name
rank
instructor
office
address
department
department
title
student
take-course
course
teach
session
offering
semester classroom
CMPT 354: Database I -- ER Design (2)
description
time
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Adding Information
• More information about departments should be kept
– Name, location, chair
name
location
chair
title
description
department
home-dept
name
course
address
member-of
major
instructor
name
rank
student
take-course
teach
session
office
CMPT 354: Database I -- ER Design (2)
semester classroom
offering
time
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Using Classes
• Current students and alumni
name
location
home-dept
chair
title
description
graduate
department
name
address
course
isa
alumni
member-of
major
instructor
name
rank
student
current_student
take-course
teach
session
office
CMPT 354: Database I -- ER Design (2)
semester classroom
offering
time
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UML
• UML: Unified Modeling Language
• UML has many components to graphically
model different aspects of an entire software
system
• UML Class Diagrams correspond to E-R
Diagram, but several differences
CMPT 354: Database I -- ER Design (2)
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UML Class Diagram Notation
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Summary
• Model real world data using entities and
relationships
• The ER model
• ER diagrams
– Entities, relationships, attributes
– Constraints, keys, cardinalities
– Weak entities
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To-Do List
• Read Chapter 6 (except for Section 6.9)
• Draw the ER diagram for the Mini_TPC
database
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