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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 E-R 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. Attributes are the characteristics, components or properties of entities.

• Diamonds are used to model the structural associations that exist between entities.

Course

CID

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 1222 123-12-2244

Kathy 2223 223-22-2245

Steve 3222 723-12-2244

Customer (Entity Set)

John Manager

Kathy Teller

Steve 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

ss#student-id addressname

STUDENT entity

Attributes

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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)

street city state zip

street-number street-name apartment-number

Composite

attribute

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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 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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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

019-28-3746

Smith North Rye

555-55-5555

Jackson Dupont Woodside

321-12-3123

Curry North Rye

321-12-3123

Adam Spring Pittsfield

L-17 1000

L-23 2000

L-14 1500

L-19 500

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

employee-nametelephone-number

employee-id

works-formanager

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

DATEPERSON

earned PERSON earned DEGREE on 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 ratiosØExpress the number of entities to which another entity can be associated via a relationship set ØAre 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:Ø One to oneØ One to manyØ Many to oneØ 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.

A3

B2

B1

B3

A1

A2

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MAPPING CARDINALITIES

• One to manyAn 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.

A3

B3B2

B4

A1

A2

B1

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MAPPING CARDINALITIES

• Many to oneAn 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.

A3

B2

B1

B3

A1

A2

A1

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MAPPING CARDINALITIES

• Many to manyAn 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.

A3

B2

B1

B3

A1

A2

A1

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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:Ø Super keyØ Candidate keyØ 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 Account- then 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 123-12-2244

Kathy 223-22-2245

Steve 723-12-2244

Customer

A-101

A-215

A-102

Account

24 May 2002

3 June 2002

20 June 2002

21 June 2002

10 May 2002

Depositor

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DESIGN ISSUE

• Use of Entity Sets versus Attributes

Consider the entity set employee with attributes employee-name 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

• Treating a telephone as an entity408-888-3565 cell-phone

408-345-5366 video-phone

408-222-3455 cell-phone

714-555-6366 cell-phone

John 408-345-5366Kathy 408-222-3455Steve 714-555-6366

EMPLOYEE (employee-name, telephone-number)

John

Kathy

Steve

Alex

TELEPHONE(telephone-number, type)EMPLOYEE (employee-name)

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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

Entity B

Relationship

Attribute

Entity C

Entity A

Attribute

Attribute

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Classification of Constraints

1. Keys

2. Single-value constraints

3. Multi-valued constraints

4. 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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StarsStars-in

year

Studios

Movies

length

Title AddressName

ownsfileType

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 dash-underlined.

• e.g., primary key for DEPENDENT: {Employee No., DName}.

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EmpolyeeEMP_DEP

GenderDName

Emp No.

Name

Dependent

Emp No.

*

Detailed Conceptual Design

EmployeeEmployee

Emp No unique identifier of an emp. IdentifierEmp Name name of an employee Composite First Name first name of an employee Mid Initials middle initials of an employee Last Name last name of an employeeNID national id of an employee UniqueAddress address of an employeeSalary salary of an employeeGender sex of an employeeDOB birth date of an employee

Detailed Conceptual Design

Emp No

Emp Name

NID

Salary

Address

Employee

GenderDOB

First Name

Mid Initials

Last Name

Employee

works

Department

Project

manage

controlworks on

supervise

Dependent

has

Depd Name

GenderDOBRelation

Emp No

Emp NameNID

Salary

Address

Gender

DOB

First Name

Mid Initials

Last Name

Proj No

Proj NameLocation

Dept No

Dept Name

Location

Employees

Phone

Hours

Start d

Conceptual Design with Weak Entities

Employee

works

Department

Project

manage

controlworks on

supervise

Dependent

has