chapter 2 data entity relationship

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DEFINE THE ENTITY

IDENTIFY THE ENTITY TYPES AND SETS OF ENTITY

EXPLAIN THE ATTRIBUTE AND A VALUE SET

EXPLAIN E-R MODEL

EXPLAIN THE FOLLOWING RELATIONSHIPTYPES 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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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 thatexists conceptually such as an event or concept

in a user environment.


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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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Recursive Entity

-A recursive entity is one in which a relation canexist between occurrences of the same entityset. This occurs in a unary relationship.

Composite Entity-If a Many to Many relationship exist we mustcreate a bridge entity to convert it into 1 to

Many. Bridge entity composed of the primarykeys of each of the entities to be connected. Thebridge entity is known as a composite entity. Acomposite entity is represented by a diamondshape with in a rectangle in an ER Diagram.


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Strong Entity Type

Entity type that is not existence-dependent on

some other entity type.

Weak Entity Type Entity type that is existence-dependent on some

other entity type.


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Entity Set

An entity may be concrete such as a person,book etc or may be abstract such asaccount,loan etc. The ER model refers to a

specific table row as an entity instance or entityoccurrence. Collection of similar entities (EntitySet) often corresponds to a table. Each entity sethas a key.All entities in an entity set have thesame set of attributes. Thus entity set is a set of

entities of the same type that share the sameproperties or attributes. An entity is representedby a rectangle containing the entity name, whichis a 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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Simple Attribute has only one component,exist independently and cannot be brokenup. ( name or gender )

Composite Attribute comprises of manycomponents, each one existingindependently. (address with sub attributes such as house_no, road_no,

town, postcode)


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

Multi-Valued Attribute is an attribute

consisting of many values. ( telephone no)Derived Attribute is 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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TYPES IN

ER MODEL

ONE TO

ONEONE TO

MANYMANY TO

MANY


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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; a

BUILDING is covered by one ROOF.


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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.


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EXAMPLE II

In the one-to-many relationship a loan is

associated with at most one customer viaborrower, a customer is associated with

several (including 0) loans via borrower


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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.


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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 chemical and 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 TotalCost that we cannot

appropriately associate

with chemical or supplier.


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chapter 2 data entity relationship

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