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Data Modeling Using the
Entity-Relationship (ER)
Model
Outline
Phases in Database Design
Example Database Application (COMPANY)
ER Model Concepts
Entities and Attributes
Entity Types, Value Sets, and Key Attributes
Relationships and Relationship Types
Weak Entity Types
Roles and Attributes in Relationship Types
ER Diagrams - Notation
ER Diagram for COMPANY Schema
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Phases in Database Design
Example: COMPANY Database
Requirements of the Company (oversimplified
for illustrative purposes)
The company is organized into DEPARTMENTs.
Each department has a name, number and an
employee who manages the department. We
keep track of the start date of the department
manager.
Each department controls a number of
PROJECTs. Each project has a name, number
and is located at a single location.
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Example COMPANY Database
(Cont.)
We store each EMPLOYEE’s social security
number, address, salary, sex, and birthdate.
Each employee works for one department but
may work on several projects. We keep track of
the number of hours per week that an employee
currently works on each project. We also keep
track of the direct supervisor of each employee.
Each employee may have a number of
DEPENDENTs. For each dependent, we keep
track of their name, sex, birthdate, and
relationship to employee.
ER Model Concepts
Entities and Attributes
Entities are specific objects or things in the mini-world that
are represented in the database. For example the
EMPLOYEE John Smith, the Research DEPARTMENT, the
ProductX PROJECT
Attributes are properties used to describe an entity. For
example an EMPLOYEE entity may have a Name, SSN,
Address, Sex, BirthDate
A specific entity will have a value for each of its attributes.
For example a specific employee entity may have
Name='John Smith', SSN='123456789', Address ='731,
Fondren, Houston, TX', Sex='M', BirthDate='09-JAN-55‘
Each attribute has a value set (or data type) associated with
it – e.g. integer, string, subrange, enumerated type, …
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Types of Attributes (1)
Simple
Each entity has a single atomic value for the attribute. For
example, SSN or Sex.
Composite
The attribute may be composed of several components. For
example, Address (Apt#, House#, Street, City, State,
ZipCode, Country) or Name (FirstName, MiddleName,
LastName). Composition may form a hierarchy where some
components are themselves composite.
Multi-valued
An entity may have multiple values for that attribute. For
example, Color of a CAR or PreviousDegrees of a STUDENT.
Denoted as {Color} or {PreviousDegrees}.
Types of Attributes (2)
In general, composite and multi-valued
attributes may be nested arbitrarily to any
number of levels although this is rare. For
example, PreviousDegrees of a STUDENT
is a composite multi-valued attribute
denoted by {PreviousDegrees (College,
Year, Degree, Field)}.
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Entity Types and Key Attributes
Entities with the same basic attributes are grouped or
typed into an entity type. For example, the EMPLOYEE
entity type or the PROJECT entity type.
An attribute of an entity type for which each entity must
have a unique value is called a key attribute of the
entity type. For example, SSN of EMPLOYEE.
A key attribute may be composite. For example,
VehicleTagNumber is a key of the CAR entity type with
components (Number, State).
An entity type may have more than one key. For
example, the CAR entity type may have two keys:
VehicleIdentificationNumber (popularly called VIN) and
VehicleTagNumber (Number, State), also known as
license_plate number.
ENTITY SET corresponding to the
ENTITY TYPE CAR
CAR
Registration(RegistrationNumber, State), VehicleID, Make, Model, Year, (Color)
car1
((ABC 123, TEXAS), TK629, Ford Mustang, convertible, 1999, (red, black))
car2
((ABC 123, NEW YORK), WP9872, Nissan 300ZX, 2-door, 2002, (blue))
car3
((VSY 720, TEXAS), TD729, Buick LeSabre, 4-door, 2003, (white, blue))
.
.
.
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SUMMARY OF ER-DIAGRAM
NOTATION FOR ER SCHEMAS
Symbol Meaning
ENTITY TYPE
WEAK ENTITY TYPE
RELATIONSHIP TYPE
IDENTIFYING RELATIONSHIP TYPE
ATTRIBUTE
KEY ATTRIBUTE
MULTIVALUED ATTRIBUTE
COMPOSITE ATTRIBUTE
DERIVED ATTRIBUTE
E1 R E2 TOTAL PARTICIPATION OF E2 IN R
N CARDINALITY RATIO 1:N FOR E1:E2 IN R
E1 R E2
(min,max) STRUCTURAL CONSTRAINT (min, max) ON PARTICIPATION
R E
OF E IN R
ER DIAGRAM – Entity Types are:
EMPLOYEE, DEPARTMENT, PROJECT, DEPENDENT
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Relationships and Relationship Types (1)
A relationship relates two or more distinct entities with
a specific meaning. For example, EMPLOYEE John Smith
works on the ProductX PROJECT or EMPLOYEE Franklin
Wong manages the Research DEPARTMENT.
Relationships of the same type are grouped or typed
into a relationship type. For example, the WORKS_ON
relationship type in which EMPLOYEEs and PROJECTs
participate, or the MANAGES relationship type in which
EMPLOYEEs and DEPARTMENTs participate.
The degree of a relationship type is the number of
participating entity types. Both MANAGES and
WORKS_ON are binary relationships.
Example relationship instances of the WORKS_FOR
relationship between EMPLOYEE and DEPARTMENT
EMPLOYEE WORKS_FOR DEPARTMENT
e1
r1 d1
e2 r2
e3 d2
r3
e4
r4
e5 d3
r5
e6
e7 r6
r7
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Example relationship instances of the WORKS_ON
relationship between EMPLOYEE and PROJECT
r9
e1
r1 p1
e2 r2
e3 p2
r3
e4
r4
e5 p3
r5
e6
e7 r6
r7
r8
Relationships and Relationship Types (2)
More than one relationship type can exist with the same
participating entity types. For example, MANAGES and
WORKS_FOR are distinct relationships between
EMPLOYEE and DEPARTMENT, but with different
meanings and different relationship instances.
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ER DIAGRAM – Relationship Types are:
WORKS_FOR, MANAGES, WORKS_ON, CONTROLS,
SUPERVISION, DEPENDENTS_OF
Weak Entity Types
An entity that does not have a key attribute
A weak entity must participate in an identifying
relationship type with an owner or identifying entity
type
Entities are identified by the combination of:
A partial key of the weak entity type
The particular entity they are related to in the
identifying entity type
Example:
Suppose that a DEPENDENT entity is identified by the
dependent’s first name and birthrate, and the specific
EMPLOYEE that the dependent is related to.
DEPENDENT is a weak entity type with EMPLOYEE as
its identifying entity type via the identifying
relationship type DEPENDENT_OF
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Weak Entity Type is: DEPENDENT
Identifying Relationship is: DEPENDENTS_OF
Constraints on Relationships
Constraints on Relationship Types
Cardinality Ratio
Specifies the maximum number of relationship instances
that an entity can participate in.
One-to-one (1:1)
One-to-many (1:N) or Many-to-one (N:1)
Many-to-many
Participation
Also called minimum cardinality constraint
Specifies the minimum number of relationship
instances that each entity can participate in.
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Many-to-one (N:1) RELATIONSHIP
EMPLOYEE WORKS_FOR DEPARTMENT
e1
r1 d1
e2 r2
e3 d2
r3
e4
r4
e5 d3
r5
e6
e7 r6
r7
Many-to-many (M:N) RELATIONSHIP
r9
e1
r1 p1
e2 r2
e3 p2
r3
e4
r4
e5 p3
r5
e6
e7 r6
r7
r8
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Relationships and Relationship Types (3)
We can also have a recursive relationship type.
Both participations are same entity type in different
roles.
For example, SUPERVISION relationships between
EMPLOYEE (in role of supervisor or boss) and (another)
EMPLOYEE (in role of subordinate or worker).
In the following figure, first role participation labeled
with 1 and second role participation labeled with 2.
In ER diagram, need to display role names to
distinguish participations.
A RECURSIVE RELATIONSHIP
SUPERVISION
EMPLOYEE SUPERVISION
e1 2
1 r1
e2 2
1 r2
e3 2
1 r3
e4
2
e5 1
1 r4
e6
2
1
r5
e7
2
r6
© The Benjamin/Cummings Publishing Company, Inc. 1994, Elmasri/Navathe, Fundamentals of Database Systems, Second Edition
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Recursive Relationship Type is:
SUPERVISION
(participation role names are shown)
Attributes of Relationship types
A relationship type can have attributes; for
example, HoursPerWeek of WORKS_ON; its
value for each relationship instance
describes the number of hours per week
that an EMPLOYEE works on a PROJECT.
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Attribute of a Relationship Type is:
Hours of WORKS_ON
Structural Constraints –
one way to express semantics
of relationships
Structural constraints on relationships:
Cardinality ratio (of a binary relationship): 1:1, 1:N,
N:1, or M:N
SHOWN BY PLACING APPROPRIATE NUMBER ON THE
LINK.
Participation constraint (on each participating
entity type): total (called existence dependency) or
partial.
SHOWN BY DOUBLE LINING THE LINK
NOTE: These are easy to specify for Binary Relationship
Types.
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Alternative (min, max) notation for
relationship structural constraints:
Specified on each participation of an entity
type E in a relationship type R
Specifies that each entity e in E participates in
at least min and at most max relationship
instances in R
Default(no constraint): min=0, max=n
Must have minmax, min0, max 1
Derived from the knowledge of mini-world
constraints
Alternative (min, max) notation for relationship
structural constraints:
Examples:
A department has exactly one manager and an
employee can manage at most one department.
– Specify (0,1) for participation of EMPLOYEE in
MANAGES
– Specify (1,1) for participation of DEPARTMENT
in MANAGES
An employee can work for exactly one department
but a department can have any number of
employees.
– Specify (1,1) for participation of EMPLOYEE in
WORKS_FOR
– Specify (0,n) for participation of DEPARTMENT
in WORKS_FOR
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The (min,max) notation
relationship constraints
(0,1) (1,1)
(1,1) (1,N)
COMPANY ER Schema Diagram
using (min, max) notation
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Acknowledgement
Reference for this lecture is
Ramez Elmasri and Shamkant B. Navathe,
Fundamentals of Database Systems,
Pearson Education.
The authors and the publishers are
gratefully acknowledged.
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