ER Model:
Introduction
- ER Model is known as the
Entity-Relationship Model. It means a
model that is used to describe and define the overall structure of any
database by using schematic and diagrammatic representation. ER Model
defines the logic of the database.
- The schematic and
diagrammatic representation is generated using certain components including
entity, attributes, relationships, etc. These components are discussed in
the next section.
ER Model:
Components
- The components of
the entity-relationship model are the building blocks which helps in the
generation of an ER Model diagram, which finally results in the design of
the logical structure of a database. There are three basic components of the
Entity-Relationship Model. These are discussed below.
- All these components have definite diagrammatical representations that are used for the generation of
ER Diagram. Let’s discuss them in detail now.
ER Model: Components
1. Entity
- An entity can be called as the basic object
which represents the ER Model. It can be a real-world object/thing present
physically or can be an object/thing present logically/virtually. Entities
are objects or things or articles or pieces that have an existence without
any dependence on any other object.
For example: Bank account can be an entity, Loan can be an
entity, Bank branch can be an entity, etc. These entities have their own
properties which are known as attributes.
2. Attribute
- Once the entities
are defined, their properties support them and are known as attributes.
These attributes also consist of specific values corresponding to the type
of entity they are related to. Attributes describe the properties and
characteristics of the entity.
For example: Attributes of Bank account (Which is an entity) can
be “ Account No, Account Holder Name, Balance, etc.”. Also, the values of
attributes are domain-specific. It means, if the account is an entity then
the balance cannot be negative. There exist six types of attributes for
specific types of entities.
a) Simple
Attribute: Simple attribute are those attributes which cannot be
divided further because of their atomic nature.
Let’s make it clear with
an example. The account number of a customer can
be a simple attribute as it cannot be divided further. The date of birth of
a person is another example of a simple attribute.
b) Composite
Attribute: Composite attributes are those attributes that are the
combination of two or more than two simple attributes joined
together.
For example: Name of a customer of the
account holder can be a composite attribute.
Ganesh + Dutt + Upreti
→ First Name + Middle Name + Last Name {Composite
Attributes}
c) Derived
Attribute: Derived Attributes are those attributes whose values are
derived from the already stored attributes in the database.
For example: Database consisting of the height of a class. Then
the value of average height can be derived from the already available data
of students.
d) Single
Valued Attribute: Single valued attribute are those which contains
only a single specified value or a single unique value.
For example: Aadhar Number, SSN No, etc.
e) Multi-Valued
Attribute: Multi-valued attributes are those which can have multiple
values against them.
For example: Address, email, Phone number are multi-valued
attributes.
f) Null Value
Attribute: Null value attributes are those which can be left blank
without passing any value.
For example : A person can have a name
without a middle name and hence middle name field can be left blank or
null.
Ganesh + Upreti
→ First Name + _______ + Last Name
3.
Relationship
- The relationship is
the third and last component of the ER Model. It is responsible for
establishing a link of communication between attributes and entities as per
requirements.
- For example: “Has an” can
be termed as a relationship between a customer and an account in the
bank.
ER Model: Relationship
- A
relationship can be categorized into three majors :
- Binary Relationship: When any
relationship is established among 2 entities, it is called a binary
relationship.
- Ternary Relationship: When any
relationship is established among 3 entities, it is called a ternary
relationship.
- n-Degree Relationship: When any
relationship is established among any number of entities(2 or more
than two), then n is replaced with that number. For example: 2-Degree, 3-Degree, 4-Degree,
etc.