Entity relationship Model (E-R Model)
It is the graphical representation of the database logic and includes
a detailed description of all entities, relationship and constraints. It
develops a conceptual design for the database. It also develops a very
simple and easy to design view of data.
In ER modeling, it describes the structure of a
database with the help of a diagram called an entity-
relationship diagram.
An Entity may be an object with a physical existence –
a particular person, car, house, or employee – or it may be an
object with a conceptual existence – a company, a job, or a
university course. An entity may be any object, class, person
or place. Set of all entities is called as entity set.
e.g.; E1 is an entity having Entity Type Student and set of
all students is called Entity Set. In the ER diagram, an entity
can be represented as rectangles.
Weak Entity:
An entity that cannot be uniquely identified by its own attributes and relies
on the relationship with other entity is called weak entity. The weak entity
doesn't contain any key attribute of its own. The weak entity is represented
by a double rectangle.
Attributes are the properties which define the entity type. For
example, Roll_No, Name, DOB, Age, Address, Mobile_No are
the attributes which defines entity type Student.
In ER diagram, attribute is represented by an oval.
, Key Attribute –
The attribute which uniquely identifies each entity in the entity
set is called key attribute. For example, Roll_No will be unique
for each student. In ER diagram, key attribute is represented by an oval with
underlying lines.
Composite Attribute –
An attribute composed of many other
attribute is called as composite
attribute. For example, Address
attribute of student Entity type consists
of Street, City, State, and Country. In
ER diagram, composite attribute is
represented by an oval comprising of
ovals.
Multivalued Attribute –
An attribute consisting more than one value for a given entity. For example,
Phone_No (can be more than one for a given student). In ER diagram, multivalued
attribute is represented by double oval.
Derived Attribute –
An attribute which can be derived from other attributes of the entity
type is known as derived attribute. e.g.; Age (can be derived from
DOB). In ER diagram, derived attribute is represented by dashed
oval.
The complete entity type Student with its attributes can be represented as:
It is the graphical representation of the database logic and includes
a detailed description of all entities, relationship and constraints. It
develops a conceptual design for the database. It also develops a very
simple and easy to design view of data.
In ER modeling, it describes the structure of a
database with the help of a diagram called an entity-
relationship diagram.
An Entity may be an object with a physical existence –
a particular person, car, house, or employee – or it may be an
object with a conceptual existence – a company, a job, or a
university course. An entity may be any object, class, person
or place. Set of all entities is called as entity set.
e.g.; E1 is an entity having Entity Type Student and set of
all students is called Entity Set. In the ER diagram, an entity
can be represented as rectangles.
Weak Entity:
An entity that cannot be uniquely identified by its own attributes and relies
on the relationship with other entity is called weak entity. The weak entity
doesn't contain any key attribute of its own. The weak entity is represented
by a double rectangle.
Attributes are the properties which define the entity type. For
example, Roll_No, Name, DOB, Age, Address, Mobile_No are
the attributes which defines entity type Student.
In ER diagram, attribute is represented by an oval.
, Key Attribute –
The attribute which uniquely identifies each entity in the entity
set is called key attribute. For example, Roll_No will be unique
for each student. In ER diagram, key attribute is represented by an oval with
underlying lines.
Composite Attribute –
An attribute composed of many other
attribute is called as composite
attribute. For example, Address
attribute of student Entity type consists
of Street, City, State, and Country. In
ER diagram, composite attribute is
represented by an oval comprising of
ovals.
Multivalued Attribute –
An attribute consisting more than one value for a given entity. For example,
Phone_No (can be more than one for a given student). In ER diagram, multivalued
attribute is represented by double oval.
Derived Attribute –
An attribute which can be derived from other attributes of the entity
type is known as derived attribute. e.g.; Age (can be derived from
DOB). In ER diagram, derived attribute is represented by dashed
oval.
The complete entity type Student with its attributes can be represented as:
