ENTITY-RELATIONSHIP MODEL
POINTS TO NOTE :
qwhat is entity-relationship diagram.
qWhy use ER
qSymbols used in ER diagram.
qComponents of ER diagram.
qTypes of attribute.
qTypes of relationship
What is entity-relationship diagram?
An ER diagram/model describes the relationship of
entities that need to be used in a database. It is
mainly a structural design for the database. It is a
framework using specialized symbols to define the
relationship between entities. It is widely used for
database designs.
ER is created based on three main components
1.Entities
2.Attributes
3.Relationship
Example of ER diagramA school needs there students record to be stored
digitally. They approach an IT company in that
regard, the IT company request an ER diagram from
the school authorities. Cous
Stu Stu_n Stu_ Cous_ _
_ID ame age ID nam
e
STUDE STU COURS
NT DY ES
Why use ER diagram ?
Entities Attributes
1. Helps to conceptualize the database and also
helps to know which fields needs to be
embedded for a particular entity.
2. Gives better understanding of the information
to be stored in database.
3. Reduces complexity and saves time.
Symbols used in ER diagram
Use
dotted
lines
Entity Multi valued Attribute Relationship Weak entity derived attribute
Components of ER diagram
ATTRIBUT RELATIONS
ENTITY
ES HIP
• Strong • key • One to One
entity • Composite • One to Many
• Weak entity • Multi-valued • Many to One
• derived • Many to Many
ENTITY It is the collection of related attribute. It can be a thing,
place, person, unit or any item about which the data
should be captured and stored in the form of properties.
It is represented as a rectangle.
STUDEN STUD
COURSE
T Y
ATTRIBUTE An attribute describes the properties of an entity. It is
represented as oval in an ER diagram.
NAM
S_ID
E
STUDEN ADDRE
AGE
T SS
CHARACTERISTICS OF AN
ATTRIBUTE
KEY ATTRIBUTEThis uniquely identifies an entity from an entity set. The
text of the attribute is underlined. E.g.. Student_ID can
uniquely identify a student from a set of students.
What is a Strong Entity?
A strong entity is an entity that is not dependent on any other entity. It has a
primary key, or a table includes a primary key.
What is a Weak Entity?
Unlike a strong entity, a weak entity is not independent as it is reliant on
another strong entity. It has to rely on other entities to ensure its existence.
S.No. Strong Entity Weak Entity
1 It is independent in nature. It depends on another strong entity.
2 It has a primary key. It does not have any primary key.
3 It is depicted by a sole rectangle. It is depicted by a dual rectangle.
4 The connection between two The connection between a weak and a
strong entities is shown by a sole strong entity is shown by a dual
diamond. diamond.
This is an attribute that is composed of
COMPOSITE ATTRIBUTE
other attributes. The composite attribute is
represented with an oval, an the other
Fnam
attributes
Lnam isM-
further connected to it with
e ovals.
e name
NAM
S_ID
E
STUDEN ADDRE
AGE
T SS
MULTI-VALUED ATTRIBUTEThis is an attribute that can possess more
than one value. It makes use of double oval.
NAM
S_ID
E
STUDEN PHON
AGE E
T
DERIVED ATTRIBUTEThis is an attribute that can be extracted
from other attribute of the entity. It is
represented with dashed oval.
NAM
S_ID
E
STUDEN
AGE DOB
T
RELATIONSHIP
A relationship is used to show the relation among entities. It makes use
of diamond shape.
TYPES OF RELATIONSHIP
ONE TO ONE: When a single element of an entity is associated with
a single element of another entity. For example. A
student has ONLY ONE identification card. And ONE
identification card is given to ONE student
STUDENT 1 HAS 1 ID CARD
ONE TO MANY: When a single element of an entity is associated
with more than one element of another entity. For
example. A customer can place MANY orders but an
order cannot be place by many customers.
CUSTOME 1 PLA M ORDER
R CE
MANY TO MANY:When more than one element of an entity is
associated with more than one element of another
entity. For example, employee can be assigned to
many projects and a project can have many employee.
ASSI
M M
EMP GNE PROJECT
D
CLASS ACTIVITY Draw an ER diagram of a bank database,
where each bank can have multiple branches,
and each branch can have multiple accounts
