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E-R Model
• The E-R model is not intended to be
associated with any particular database
model.
• E-R diagrams are intended to allow humans
the ability to capture more of the
application’s meaning.
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History of E-R Model
• E-R Model was proposed
by Dr. Peter Chen
(currently professor at
Louisiana State University)
• Chen’s original paper on ER Model is the 35th most
sited paper in computer
science
• Chen has written papers
interconnecting E-R model
and linguistics
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The Entity-Relationship Model
(History)
• Developed by Peter Chen in the 1970’s
• Several variations have evolved
• All are designed towards the concise
capture of the application semantics in
terms appropriate for subsequent mapping
to a specific database model.
• It is currently the most widely used.
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The Entity-Relationship Approach
• Entity: an object that exists and is
distinguishable from other objects. i.e.
person, place, thing, event or concept about
which information(attributes) is recorded.
The basic unit of the E-R model.
• The structure of an entity is called its
schema.
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More Terminology
• Object: things in the real world that can be
observed and classified because they have
related properties
• Entity: the groupings we use when we
categorize the objects. Sometimes called a
class.
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Three Basic Graphical Symbols in an
ER Diagram
• Rectangles are used to model conceptual
data units or data objects.
• Circles are used to model attributes.
Course
Attributes are the characteristics,
components or properties of entities.
CID
• Diamonds are used to model the structural
associations that exist between entities.
Enroll
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BASIC CONCEPTS
There are 3 basic notions in the E-R Model:
Entity Sets
Relationship Sets
Attributes
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AN ENTITY
An entity is a “thing” or “object” in the real world that
is distinguishable from all other objects. It has an
unique set of properties that may uniquely identify an
entity. For example, a student entity has three
attributes: name , student-id, and social-security
numbers.
John 1222 123-12-2244
STUDENT entity
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ENTITY SETS
An entity set is a set of entities that share the same
properties or attributes. Entity sets do not need to be
disjoint. For example, a customer can also be an
employee.
John
Kathy
Steve
1222
2223
3222
123-12-2244
223-22-2245
723-12-2244
Customer (Entity Set)
John
Kathy
Steve
Manager
Teller
Teller
Employee (Entity Set)
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ATTRIBUTE
An entity is represented by a set of attributes.
Attributes are descriptive properties possessed by each
member of an entity set.
Example:
STUDENT entity
Attributes
name
STUDENT
student-id
address
ss#
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ATTRIBUTE TYPES
There are several different types of attributes.
Simple and Composite
Single-valued and Multivalued
Derived
Null
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ATTRIBUTE TYPES
Simple attributes are not divided into subparts.
Composite attributes can be divided into subparts.
Using composite attributes in a design schema is a
good choice if a user will wish to refer to an entire
attribute on some occasions.
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ATTRIBUTE TYPES
Example:
address (Composite attribute)
Composite
street
city
state
zip
attribute
street-number
street-name
apartment-number
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ATTRIBUTE TYPES
Single-valued attributes are attributes that only have a
single value for a particular entity.
Multi-valued attributes refers to entities that are not singledvalue and Null valued. For example, consider an employee
entity set with the attribute phone-number. An employee may
have zero, one, or several phone numbers, different employee
may have different numbers of phones.
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ATTRIBUTE TYPES
Null attribute is used when an entity does not have a value
for an attribute.
Derived attributes refer to an attribute that can be derived
from other related attributes or entities. For instance, suppose
that Age and Date-of-birth are attributes of the CUSTOMER
entity set. We can calculate Age from Date-of-birth. In this
case, Age is a derived attribute.
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RELATIONSHIP SETS
A relationship is an association among several entities.
A relationship set is a set of relationships of the same type. Consider
the two entity sets customer and loan. We define the relationship set
borrower to denote the association between customers and bank loans
that the customers have.
321-12-3123
Jones
Main
Harrison
L-17
1000
019-28-3746
Smith
North
Rye
L-23
2000
555-55-5555
Jackson Dupont
Woodside
L-14
1500
321-12-3123
Curry
North
Rye
L-19
500
321-12-3123
Adam
Spring
Pittsfield
L-16
1300
customer
loan
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RECURSIVE RELATIONSHIP
Recursive relationship- the same entity set participates in a
relationship set more than once, in different roles. The function that an
entity plays in a relationship is called that entity’s role. For example,
consider an entity set employee that records information about all
employees of the bank. We may have a relationship set works-for that is
modeled by ordered pairs of employee entities. The first employee of a
pair takes the role of worker, whereas the second takes the role of
manager.
employee-name
telephone-number
employee-id
manager
works-for
employee
worker
Recursive Relationship
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BINARY RELATIONSHIP
SETS
Binary relationship set - relationship that involves
two identity sets.
Most of the relationship sets in a database system
are binary. Occasionally, however, relationship set
involve more than two entity.
OWNER
Owns
PROPERTY-FOR-RENT
Binary relationship called Owns
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TERNARY RELATIONSHIP
SET
Ternary relationship set - relationship that involves
three identity sets
DEGREE
earned
PERSON
PERSON earned DEGREE on DATE
DATE
Ternary Relationship Set
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DEGREE OF A
RELATIONSHIP SET
Degree of a relationship set is the number of entity
sets that participate in a relationship set. For example,
a binary relationship set is of degree 2; a ternary
relationship set is of degree 3.
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CONSTRAINTS
An E-R enterprise scheme may define certain constraints to
which the contents of a database must conform. The two of
most important types of constraints are Mapping Cardinalities
and Participation Constraints.
Participation Constraints
The participation of an entity set E in a relationship set R is
said to be total, if every entity in E participates in at least one
relationship in R. If only some entities in E participate in
relationship R, the participation of entity set E in relationship
R is said to be partial.
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CONSTRAINTS
Mapping Cardinalities or Cardinality ratios
Express the number of entities to which another entity can be
associated via a relationship set
Are most useful in describing binary relationship sets. For a
binary relationship set R between entity sets A and B, the
mapping cardinality must be one of the following:
 One to one
 One to many
 Many to one
 Many to many
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MAPPING CARDINALITIES
One to one
An entity in A is associated with at most one entity in
B, and an entity in B is associated with at most one
entity in A.
A1
B1
A2
B2
A3
B3
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MAPPING CARDINALITIES
One to many
An entity in A is associated with any number (zero or more) of
entities in B. An entity in B, however, can be associated with
at most one entity in A.
A2
B1
B2
B3
A3
B4
A1
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MAPPING CARDINALITIES
Many to one
An entity in A is associated with at most one entity in B. An
entity in B, however, can be associated with any number (zero
or more) of entities in A.
A1
A1
B1
A2
A3
B3
B2
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MAPPING CARDINALITIES
Many to many
An entity in A is associated with any number (zero or more) of
entities in B, and an entity in B is associated with any number
(zero or more) of entities in A.
A1
A1
A2
A3
B1
B2
B3
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KEYS
A key allows us to identify a set of attributes that
suffice to distinguish entities from each other. Keys
also help uniquely identify relationships, and thus
distinguish relationships from each other. There are
three types of keys:
Super key
Candidate key
Primary key
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KEY
Superkey is a set of one or more attributes that, taken collectively, us
to identify uniquely an entity in the entity set. For example, customer-id
is a superkey.
Candidate key is a minimal superkey. For example, customer-name
and customer-street is sufficient to distinguish among members of the
customer entity set. Then {customer-name, customer-street} is a
candidate key .
Primary key denotes a candidate key that is chosen by the database
designer as the principal means of identifying entities within an entity
set. the primary key should be chosen such that its attributes are never,
or very rarely, changed. For example, Social-security numbers are
guaranteed to never changed.
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RELATIONSHIP SETS
Primary key of an entity set allows us to distinguish
among entities of the set. Similar mechanism is needed
in order to distinguish among the various relationships
of a relationship set.
The structure of the primary key for the relationship
set depends on the mapping cardinality of the
relationship set.
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RELATIONSHIP SETS
For example, suppose that there is a relationship set
Depositor, with attribute access-date, between
Customer relation and Account relation. Suppose that
the relationship set is many to many. Then the
primary key of Depositor consists of the union of the
primary keys of Customer and Account. If a customer
can have only one account- that is, if the Depositor
relation is many to one from Customer to Accountthen the primary key of the Depositor relationship is
simply the primary key of customer.
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RELATIONSHIP SETS
Customer (ss# , name)
Account (account-num)
Depositor (ss#, account-num, access-date)
John
Kathy
123-12-2244
223-22-2245
10 May 2002
24 May 2002
723-12-2244
Customer
A-215
3 June 2002
20 June 2002
Steve
A-101
21 June 2002
Depositor
A-102
Account
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DESIGN ISSUE
Use of Entity Sets versus Attributes
Consider the entity set employee with attributes employeename and telephone-number.
Treating a telephone as an attribute telephone-number
implies that employees have precisely one telephone number
each.
Treating a telephone as an entity telephone permits
employees to have several telephone number ( including zero)
associated with them. However, we could instead easily
define telephone-number as a multivalued attribute to allow
multiple telephones per employee.
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USE OF ENTITY SETS
VERSUS ATTRIBUTES
Treating a telephone as an attribute
John
408-345-5366
Kathy
408-222-3455
Steve
714-555-6366
EMPLOYEE (employee-name, telephone-number)
Treating a telephone as an entity
John
408-888-3565
cell-phone
Kathy
408-345-5366
video-phone
Steve
408-222-3455
cell-phone
Alex
714-555-6366
cell-phone
EMPLOYEE (employee-name)
TELEPHONE(telephone-number, type)
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ENTITY-RELATIONSHIP
DIAGRAM
Ellipses, which represent attributes
Diamonds, which represent relationship sets
Line, which link attributes to entity sets and entity sets to
relationships sets
Double ellipses, which represent multivalued attributes
Double lines, which indicate total participation of an entity
in a relationship set
Double rectangles, which represent weak entity sets
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Example of the 3 elements in E/R
Diagram
Attribute
Attribute
Attribute
Entity
A
Entity
B
Entity
C
Relationship
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Classification of Constraints
1.
2.
3.
4.
Keys
Single-value constraints
Multi-valued constraints
Mapping Cardinalities and Participation
Constraints
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Key in the E/R Model
• Superkey is a set of one or more attributes that, taken collectively, for
us to identify uniquely an item in the entity set. For example,
customer-id is a superkey.
• Candidate key is a minimal superkey. For example, customer-name
and customer-street is sufficient to distinguish among members of the
customer entity set. Then {customer-name, customer-street } is a
candidate key.
• Primary key denotes a candidate key that is chosen by the database
designer as the principal means of identifying items within an entity
set. the primary key should be chosen such that its attributes are never,
or very rarely, changed. For example, Social-security numbers are
guaranteed to never changed.
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Single/Multi-valued attributes
• Single-valued attributes are attributes that only
have a single value for a particular entity.
• Multi-valued attributes refers to items that are not
singled-value and Null valued. For example,
consider an employee entity set with the attribute
phone-number. An employee may have zero, one,
or several phone numbers; different employee may
have different numbers of phones.
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Title
year
Movies
Name
Stars-in
Address
Stars
length
fileType
owns
Studios
Name
Address
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Mapping Cardinalities or Cardinality
ratios
• Express the number of items to which another item can be
associated via a relationship set
• Are most useful in describing binary relationship sets. For
a binary relationship set R between entity sets A and B, the
mapping cardinality must be one of the following:




One to One
One to Many
Many to One
Many to Many
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Participation Constraints
• The participation of an entity set E in a
relationship set R is said to be total, if every
item in E participates in at least one
relationship in R. If only some items in E
participate in relationship R, the
participation of entity set E in relationship R
is said to be partial.
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Weak Entity Sets
• There is an occasional condition in which
an entity set’s key is composed of attributes
some or all of which belong to another
entity set. Such an entity set is called a
weak entity set.
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Discriminator
• The discriminator of a weak entity set is a set of
attributes that allows this distinction to be made.
For example, the discriminator of a weak entity set
payment is the attribute payment-number, since,
for each loan a payment number uniquely
identifies one single payment for that loan. The
discriminator of a weak entity set is also called the
partial key of the entity set.
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Requirements for Weak Entity Sets
• We cannot obtain key attributes for a weak entity
set indiscriminately. Rather, if E is a weak entity
set then its key consists of:
 Zero
or more of its own attributes, and
 Key attributes from entity sets that are reached by
certain many-one relationship from E to other entity
sets. These many-one relationship are called
supporting relationships for E.
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Discriminator (cont.)
• Note: although each payment entity is distinct,
payments for different loans may share the same
payment-number. Thus, payment entity set does
not have a primary key; it is a weak entity set.
• The primary key of a weak entity set is formed by
the primary key of the identifying entity set, plus
the weak entity set’s discriminator.
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Identifying a Weak Entity Type
•
•
•
•
•
A Weak entity type doesn’t have a primary key.
If X is a weak entity type and Y is the entity type on
which X is dependent.
We form a primary key for X by combining the primary
key of Y which one or more attributes, called
discriminator or partial key, from X.
In an E/R Diagram, a partial key is usually dashunderlined.
e.g., primary key for DEPENDENT: {Employee No.,
DName}.
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Emp No.
DName
Name
Gender
Dependent
*
EMP_DEP
* Doted-line = double-line
Emp No.
Empolyee
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Detailed Conceptual Design
Employee
Emp No
Emp Name
First Name
Mid Initials
Last Name
NID
Address
Salary
Gender
DOB
UCSC
unique identifier of an emp. Identifier
name of an employee
Composite
first name of an employee
middle initials of an employee
last name of an employee
national id of an employee Unique
address of an employee
salary of an employee
sex of an employee
birth date of an employee
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Detailed Conceptual Design
First Name
Emp No
Emp Name
Mid Initials
Last Name
Employee
NID
Address
Salary
DOB
UCSC
Gender
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First Name
Emp No
NID
Mid Initials
Emp Name
Last Name
Address
Location
Dept Name
Phone
supervise
works
Start d
Employee
Salary
Gender
Dept No
Department
manage
Hours
has
Employees
control
works on
DOB
Dependent
Depd Name
Proj No
Project
Gender
Relation
UCSC
DOB
Location
Proj Name
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Conceptual Design with
Weak Entities
supervise
works
Department
Employee
manage
has
control
works on
Dependent
UCSC
Project
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