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Entity is an object of the real world which can

store data and can be defined with clarity. Example : something that exist physically such

as individual, place, article or something that

exists conceptually such as an event or concept

in a user environment. Each entity has attributes, or a particular

properties that describe the entities.

Example: student (entity) name Aminah has

attributes or properties of her own Student

Reg. No, name and grade

A particular value of an attribute, such as 93

for the grade is a value of the attribute.

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Sometimes, the value of an attribute is

unknown or missing and sometimes a value isnot applicable.

In such cases, the attribute can have the

special value of null.

Example : until the teacher grades anassignment , the team grade is missing or null..

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ENTITY EXAMPLE

invidual Customer, Worker, Supplier

venue / place District, State, Country

object Stationary, Furniture

event Purchase Order, Booking,Sale

concept Project, Account, Unit

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The ER model consist of different types ofentities.

The existence of an entity may depends onthe existence of one or more other entities.

In other words : an entity is said to beexistence dependent.

Entities based on their characteristic areclassified as follows :

Strong entitiesWeak entities

Recursive entities

Composite entities

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Weak Entity Type

An entity set that does not have sufficient

attribute to form a primary key is termed as

weak entity.

It is also a type that is existence-dependent onsome other entity type.

Strong Entity Type

Entity set that has a primary key is strong entity

set. Entity type that is notexistence-dependent on

some other entity type.

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Recursive Entity

-A recursive entity is one in which a relation canexist between occurrences of the same entity

set. This occurs in a unary relationship.

Unary relationship is relationship between the

instances of a single entity type. (bothparticipants in the relationship are the same

entity)

Eg: employee to manager. Ones manager is, in most

cases also an employee with a manager of his own.

EMPLOYEE

Employee_ID Name Birthdate Manager_ID

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Composite Entity-If a Many to Many relationship exist we must

create a bridge entity to convert it into 1 to

Many.

Bridge entity composed of the primary keys

of each of the entities to be connected.

The bridge entity is known as a composite

entity. A composite entity is represented by a

diamond shape with in a rectangle in an ER

Diagram.

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Eg: in a school database, a TEACHER can

teach many different SUBJECTs and aSUBJECT can be taught by many differentTEACHERs.

Such situations are modelled using a

composite entity, which is usuallydecomposed to several one to manyrelationship later in the modelling process.

The composite entity must contain theprimary keys of the associated entities as its

foreign keys. Represent using a diamond in a rectangle.

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SUBJECTTEACHER teachteach

LNameFName

Ext

Subj_code

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Entity Set

An entity may be concrete such as a person,book etc or may be abstract such as account,loan etc.

The ER model refers to a specific table row as anentity instance or entity occurrence.

Collection of similar entities (Entity Set) oftencorresponds to a table.

Each entity set has a key.

All entities in an entity set have the same set ofattributes. Thus entity set is a set of entities ofthe same type that share the same properties orattributes.

An entity is represented bya rectangle containing the entity name, which isa noun usually written in capital letters.

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A description or an entity characteristic isknown as an attribute.

Used to differentiate object in an entity.

The symbol used to represent attribute isoval and connected to entities by lines

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Explain the attribute and a value set.

Explain E-R model.

Explain the following relationship types in E-

R model : 1:1 (one to one)

1:M (one to many)

M:N (many to many)

Draw E-R model based on a given situation.

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Simple Attribute has only one component,exist independently and cannot be brokenup. Eg: name or gender

Composite Attribute comprises of manycomponents, each one existingindependently. Eg: address with sub attributes such as

o house_no,

o road_no,

o town

o postcode

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Solitary Valued Attributeis an attribute thatconsists of only a single value. (matric_no, ic_no )

Multi-Valued Attribute is an attributeconsisting of many values. ( telephone no)

Derived Attributeis an attribute where its

values is derived from the value of relatedattributes or set of other attributes.( ageattribute derived from the date of birth )

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[ composite attribute ]

[ multi-valued attribute]

[ derived attribute]

[ simple attribute]

STUDENT

matricno

age

address

dateofbirth

gender

studentname

postcode

garden

road

houseno

phoneno

specialty

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You share many relationships with members

of your family.

For instance, you and your mother are

relatedYou have only one mother, but she

may have several children.

You and your siblings are related you may

have many brothers and sisters and, of

course, they'll have many brothers and

sisters as well. If you're married, both you and your spouse

have a spouseeach otherbut only one at a

time.

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So in database, relationship is basically a set

of meaningful association between tables

A relationship exists between two database

tables when one table has a foreign key thatreferences the primary key of another table.

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E-R Model is a conceptual and semantic

model of a database that captures meanings

rather than an actual implementation.

Use E-R model to get a high-level graphicalview of essential components of your

database and how they are related

So, the E-R Model is viewed as set of

Entities Relationships among the entities

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Symbols used in E-R Diagram

Entityrectangle

Attribute oval

Relationship diamond

Link - line

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TYPES INER MODEL

ONE TOONE

ONE TOMANY

MANY TOMANY

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One to one 1 :1

This type of relationship takes place when a

single occurrence of an entity is related to just

one occurrence of a second entity.

For example, a ROOF covers one BUILDING; aBUILDING is covered by one ROOF.

In other words, both tables can have only one

record on either side of the relationship. Each

primary key value relates to only one (or no)record in the related table.

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One-to-many 1:M

Relationships occur when each record in TableA

may have many linked records in TableB but

each record in TableB may have only onecorresponding record in TableA.

This relationship is similar to the one between

you and a parent. You have only one mother, but

your mother may have several children.

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Many to manyM: N

Relationship where multiple rows from table A can

correspond to multiple rows in table B.

An example of such a relationship is a schoolwhere teachers teach students.

In most schools each teacher can teach multiple

students and each student can be taught by

multiple teachers.Other example; if you have several siblings, so do

your siblings (have many siblings).

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EXAMPLE II

A customer is associated with several (possibly 0)

loans via borrower A loan is associated with several (possibly 0)

customers via borrower

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The relationship type orderconnects

entities chemicaland supplier.

The relationship is many-to-many because

each chemical can be from several suppliers

and each supplier has a number of

chemicals.

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An order has a purchase date, amount, and

total cost as well as the chemical and

supplier information.

Thus, orderhas attributes PurchaseDate,

amount, and TotalCostthat we cannot

appropriately associate

with chemicalor supplier.

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Describe the relationship for each E-R model. Eg 1:

Situation : An employee has a single resume;

each resume belongs to only one person.

Description : The 1:1 relationship reflects the rules that

an employee can have only one resume and that

a resume can belong to only one employee.

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Eg 2:

Description: A department can have many employees,

but that each individual employee can work

for only one department.

1 M

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Eg 3:

Description: An employee can work on more than one project,

and a project can have more than one employee assigned.

M1