Ø ER model (entity
relationship model) – entity
relationship model is based on real world scenario and enities.ER model create
entity set relationship set general attribute and their constraints. ER model
is best uses for conceptual design at a database. This model is base on –
a) Entity
and their attribute
b) Relationship
among two or more entity
Here are some
shape that are used to create ER diagram –
Rectangle: Represents
Entity sets.
Ellipses:
Attributes
Diamonds: Relationship
Set
Lines: They
link attributes to Entity Sets and Entity sets to Relationship Set
Double
Ellipses: Multivalued Attributes
Dashed
Ellipses: Derived Attributes
Double
Rectangles: Weak Entity Sets
Double
Lines: Total participation of an entity in a
relationship set
A sample E-R
Diagram:
Multivalued Attributes:
An attribute that can hold multiple values is known as multivalued attribute.
We represent it with double ellipses in an E-R Diagram. E.g. A person can have
more than one phone numbers so the phone number attribute is multivalued.
Derived Attribute: A
derived attribute is one whose value is dynamic and derived from another
attribute. It is represented by dashed ellipses in an E-R Diagram. E.g. Person
age is a derived attribute as it changes over time and can be derived from
another attribute (Date of birth).
E-R diagram with multivalued and
derived attributes:
Total Participation of an Entity set:
A Total participation of an entity set represents that each entity in entity set must have at least one relationship in a relationship set. For example: In the below diagram each college must have at-least one associated Student.
A Total participation of an entity set represents that each entity in entity set must have at least one relationship in a relationship set. For example: In the below diagram each college must have at-least one associated Student.
1. Entity – A
entity is an ER models real world entity having property called attribute every
attribute is defined by its set of value called domain.
Example –
school database, a student has various attribute like – name, age, and id.
2. Relationship –
the logical association among entity is
called relationship, relationship are mapped with entities in various ways.
Mapping cardinalities define the number of association between two entities.
Relationship Set
A set of relationships of similar
type is called a relationship set. Like entities, a relationship too can have
attributes. These attributes are called descriptive attributes.
Degree of Relationship
The number of participating
entities in a relationship defines the degree of the relationship.
·
Binary = degree 2
·
Ternary = degree 3
·
n-ary = degree