Entity-Relationship Modeling

In this section you are introduced to Entity-Relationship modeling a popular way to

design databases.

Why Data Modeling?

•To understand “meaning” or “semantics” of data

•To communicate about information requirements (among the project members and customers)

•To use it as conceptual data model of database (our interest)

Understanding Data

The understanding of data involves understanding:

Each user’s perspective of the data

The nature of the data independent of its physical representation

The use of the data across user views.

Good Data Model Characteristics

SNO Characteristic

1 Structural Validity

2 Simplicity

3 Expressibility

4 Nonredundancy

5 Shareability

6 Extensibility

7 Integrity

8 Diagrammatic Representation

ER Model Concepts

Entity

Entity Type

Entity Set

Relationship

Relationship Type

Relationship Set

Week Entities

Entity

An entity is an “object” in the real world that is distinguishable from all other objects.

An entity may be a concrete or an abstract object.

Each entity may have one or more of properties.

Entity Examples

Each person in an organization is an entity. It is a concrete entity.

If the organization employs 150 persons, then it means the organization has 150 person entities.

If there is a person by name john, then john is an entity. We used the term john even though the person has first name, last name, designation, etc., because the name uniquely distinguishes the entity from the remaining 149 entities.

Entity Examples (contd...)

Similarly each department in an organization is an entity. (abstract)

Each purchase order is an entity. (concrete)

Each course in a university is an entity. (abstract)

Each university itself is also an entity. (concrete)

Each book is an entity. (concrete)

Each flight reservation is an entity. (abstract)

Entity Attributes

An attribute of an entity describes a specific characteristic or property of the entity.

For example, first name is an attribute of person entity. color is an attribute of entity car. Another attribute is price.

Depending the database requirements, you choose the attributes for entities.

Entity Attribute Examples

Attributes of some entity types are as follows:

Entities Attributes

person person id, first name, last name,

dob, address

book book id, title, author, publisher,

price

course course id, title, teacher

Entity Types

An entity type is a named set of attributes of similar entities.

Examples of entity types Person, Book and Course.

Person: {person id, first name, last name, dob, address }

Book: {book id, title, author, publisher, price}

Course: {course id, title, teacher}

Note that we used id columns (person id, book id, course id) in each entity type so that we can uniquely identify each entity.

Entity Set

An entity set is a named collection of entities having the same attributes.

Example:

An entity set for persons could be as follows:

Persons: {<1001, ‘Hari’, ‘KVS’, 1962-01-20, ‘Hyderabad’, ‘301, Banjara Hills’>,

<1002, ‘Mathews’, ‘Philips’, 1965-08-10, ‘Bangalore’, ‘10, Nagavara’>

}

This entity set has two entities.

Simple Attribute

An attribute that can not be divided into two or more components is a simple attribute.

Examples of simple attributes are:

person id, first name, last name and dob.

Composite Attribute

A composite attribute consists of multiple components where each component has its own independent existence.

An example is attribute address. An example value for address is “305, 2nd C Main Road, 8th Block, Koramangala, Bangalore” where each component has it own existence. “305” is a house number, “2nd C Main Road” is street , “8th Block” indicates small area, and “Koramangala” is an area of “Bangalore”.

Based on the application requirements, the attribute is retailed as composite attribute or split into its component simple attributes.

Single-Valued Attributes

A single-valued attribute is an attribute that can contain only single value in each entity of an entity type.

For example, attributes person id, first name, last name and dob in the following example are single valued attributes:

person- id fist name last name phone dob

1001 Akhil M 7498750138

8035621234

1997-01-18

1002 Bhargav Tamiri 9498012398 1997-03-15

1003 Krishna S 8012398654

9449010137

1990-11-25

Multi-Valued Attributes

A multi-valued attribute is an attribute that can contain multiple values in each entity of an entity type.

For example, attribute phone may have multiple phone numbers in each entity of entity type Person.

person- id fist name last name phone dob

1001 Akhil M 7498750138

8035621234

1997-01-18

1002 Bhargav Tamiri 9498012398 1997-03-15

1003 Krishna S 8012398654

9449010137

1990-11-25

Derived Attributes

A derived attribute is an attribute that has its value derived from one or more attributes not necessarily from the same entity type.

For example, attribute age in Person is a derived attribute as its value is derived from dob.

person-

id

fist

name

… phone dob age

1001 Akhil .. 7498750138

8035621234

1997-01-18 16

1002 Bhargav … 9498012398 1997-03-15 16

1003 Krishna … 8012398654

9449010137

1990-11-25 23

Domain

A domain is a set of values an attribute of an entity set can have in an entity.

Sample domains:

Attribute Domain

first name set of 30 character strings

color red, green, silver, white

price 30000 to 70000

rating 0 to 10

Keys

A key is a minimal set of attributes whose values uniquely identify an entity in an entity set.

Entity Set Key

Persons person id

Books book id

Courses course id

Student Marks roll number, subject id

Candidate Key and Primary Key

An Entity Set or Entity Type may have more than one key. Each key is known as candidate key. One of the candidate keys is designated as primary key used as the main key for the entity set.

Example: Account(customer number, PAN number, aadhaar number, first name, last name, balance)

Candidate keys: customer number, PAN number, aadhaar number

Primary key: customer number

Diagrammatic Representation of Entity Type

Entity type is represented using a rectangular box with entity type name written inside it.

Examples of Entity Types:

Person Book

Diagrammatic Representation of Entity Set

Diagram used for entity sets is the same that is used for entity types:

Examples of Entity Sets:

Persons Books

Entity Type Versus Entity Set

1. In an ER model, use either entity types or entity sets.

2. Do not use a mix of entity types and entity sets in the ER diagrams.

3. Use naming conventions consistently.

4. Though the slides use singular names for entity types and plural names for entity sets, you can use any name you want.

Diagrammatic Representation of Entity Type Attributes

Entity type attributes are represented in the bottom portion of rectangular box separated by a line from entity type name.

An examples of Entity Type with attributes:

Employee

Employee Number

First name

Last name

Date of birth

Designation

Department Name

Relationship

A relationship is an association between two or more entities.

For example, there is a borrow relationship between entity sets Persons and Books to indicate that a person (entity of entity set Persons) borrows one or more books (entities of entity set Books). We can also say the relationship between Books and Persons is borrowed by to indicate a book is borrowed by zero or one person.

Primary key values of participating entities will identify the relation.

Relationship Type

A relationship type represents collection of associations of similar type.

Similar types of associations means associations having the same attributes from participating entity types or sets. Attribute set (person id, book_id) represents a relationship type. A name that could be given to the relationship type is Borrower. The relationship type can be specified as Borrower(person id, book id).

Relationship Set

A relationship set is an instance of a relationship type. It means one or more associations of a relationship type is a relationship set.

For example relationship set Borrowers of relationship type Borrower(person id, book id) could be as follows:

{ <1001, 12956>, <1001, 23567>, <1002, 12345>}

Relationships Explained using Semantic Net

Person Book Borrows

p1

p2

p3

b1

b2

b3

b4

r1

r2

r3

Diagrammatic Representation of Relationship

Relationship between two entity types is represented with a line as shown in the following diagram:

Person Book

Borrows

Degree of Relationship Type

Number of entity types involved in a relationship type is called degree of the relationship type.

Person Book

Borrows

Borrows is a binary relationship type

Relationship Types of Various Degrees

Degree Kind of Relationship Type

2 Binary relationship type

3 Ternary relationship type

4 Quaternary relationship type

An Example of Ternary Relationship Type

Rental Agency

Tenant Landlord Arranges

An Example of Quaternary Relationship Type

Real Estate Agency

Builder Bank Arranges

Buyer

Strong and Week Entity Types

Strong Entity Type: An entity type that does not

depend on other entity types for its existence.

Example: Person

Week Entity Type: An entity type that depends

on other entity types for its existence.

Example: Hobby

Structural Constraints

Structural constraints are constraints on Entity

Types on how many entities of an Entity Type

participate in a relationship.

Identifying and representing constraints on

Entity Types is an important step in E-R modeling

of an enterprise data.

Multiplicity constraint is the main type of

constraint on relationships.

Multiplicity Constraint

Multiplicity is the number of possible entities of

an entity type that may relate to a single entity

of an associated entity type participating in a

relationship.

Binary Relationships

Binary relationships are relationships between

two entity types.

Types of binary relationships:

• One-to-one (1:1) relationships

• One-to-many (1:*) relationships

• Many-to-many (*:*) relationships

One-to-one (1:1) Relationship

A relationship is a 1:1 relationship, if an entity in

one entity type of the relationship is associated

with at most one entity in another entity type

of the relationship and vice versa.

Employee Department

Manages

Diagrammatic Representation of 1:1 Relationship

Department Employee

Manages

0..1 1..1

Multiplicity 0..1 indicates

that an employee can

manage none or one

department

Multiplicity 1..1 indicates a

department is managed by

exactly one employee

One-to-many (1:*) Relationship

A relationship between two entity types is a one-to-

many (1:*) relationship, if an entity in one entity type

is associated with any number (zero or more) of

entities in another entity type whereas an entity of the

latter entity type can be associated with at most one

entity of the former entity type.

Staff Subject Teaches

Diagrammatic Representation of 1:* Relationship

Subject Staff

Teaches

0..* 0..1

Multiplicity 0..* indicates

that a staff member teaches

none or one or more subjects

Multiplicity 0..1

indicates a subject is

taught by none or at

most one staff member

Many-to-one (*:1) Relationship

This is a mirror relationship of the one-to-many

relationship.

The one-to-many relationship of the earlier slide can be

called as many-to-one relationship if it is represented

as Taught-by relationship and swap its entity types.

Many-to-many (*:*) Relationship

A relationship between two entity types is a many-to-

many (*:*) relationship, if an entity in one entity type is

associated with any number (zero or more) of entities

in another entity type and an entity of the latter entity

type is also associated with any number entities of the

former entity type.

Author Book Wrote

Diagrammatic Representation of *:* Relationship

Book Author

Wrote

1..* 1..*

Multiplicity 1..* indicates

that an author wrote one or

more books

Multiplicity 1..* indicates

a book was written by

one or more authors

Cardinality and Participation Constraints

Cardinality is the maximum number of relationship

occurrences for an entity participating in a given

relationship type.

Participation determines whether all or only some

entities participate in a relationship.

Cardinality and Participation Constraints

Entity -2 Entity -1

Relationship-1

p..q m..n

Cardinality

Participation