January 29, 2020
Amitraj
Entity Sets -
An entity set is a set of same type of entities.
An entity refers to any object having-
-> Either a physical existence such as a particular person, office, house or car.
-> Or a conceptual existence such as a school, a university, a company or a job.
In ER diagram,
-> Attributes are associated with an entity set.
-> Attributes describe the properties of entities in the entity set.
-> Based on the values of certain attributes, an entity can be identified uniquely.
There are 2 Types of Entity Sets -
1. Strong Entity Set-
-> A strong entity set is an entity set that contains sufficient attributes to uniquely identify all its entities.
-> In other words, a primary key exists for a strong entity set.
-> Primary key of a strong entity set is represented by underlining it.
Symbols Used -
-> A single rectangle is used for representing a strong entity set.
-> A diamond symbol is used for representing the relationship that exists between two strong entity sets.
-> A single line is used for representing the connection of the strong entity set with the relationship set.
-> A double line is used for representing the total participation of an entity set with the relationship set.
-> Total participation may or may not exist in the relationship.
2. Weak Entity Set-
-> A weak entity set is an entity set that does not contain sufficient attributes to uniquely identify its entities.
-> In other words, a primary key does not exist for a weak entity set.
-> However, it contains a partial key called as a discriminator.
-> Discriminator can identify a group of entities from the entity set.
-> Discriminator is represented by underlining with a dashed line.
-> The combination of discriminator and primary key of the strong entity set makes it possible to uniquely identify all entities of the weak entity set.
this primary key is not formed by the weak entity set completely.
Symbols Used -
-> A double rectangle is used for representing a weak entity set.
-> A double diamond symbol is used for representing the relationship that exists between the strong and weak entity sets and this relationship is known as identifying relationship.
-> A double line is used for representing the connection of the weak entity set with the relationship set.
-> Total participation always exists in the identifying relationship.
Difference Between Strong & Weak Entity Set-
An entity set is a set of same type of entities.
An entity refers to any object having-
-> Either a physical existence such as a particular person, office, house or car.
-> Or a conceptual existence such as a school, a university, a company or a job.
In ER diagram,
-> Attributes are associated with an entity set.
-> Attributes describe the properties of entities in the entity set.
-> Based on the values of certain attributes, an entity can be identified uniquely.
There are 2 Types of Entity Sets -
1. Strong Entity Set-
-> A strong entity set is an entity set that contains sufficient attributes to uniquely identify all its entities.
-> In other words, a primary key exists for a strong entity set.
-> Primary key of a strong entity set is represented by underlining it.
Symbols Used -
-> A single rectangle is used for representing a strong entity set.
-> A diamond symbol is used for representing the relationship that exists between two strong entity sets.
-> A single line is used for representing the connection of the strong entity set with the relationship set.
-> A double line is used for representing the total participation of an entity set with the relationship set.
-> Total participation may or may not exist in the relationship.
2. Weak Entity Set-
-> A weak entity set is an entity set that does not contain sufficient attributes to uniquely identify its entities.
-> In other words, a primary key does not exist for a weak entity set.
-> However, it contains a partial key called as a discriminator.
-> Discriminator can identify a group of entities from the entity set.
-> Discriminator is represented by underlining with a dashed line.
-> The combination of discriminator and primary key of the strong entity set makes it possible to uniquely identify all entities of the weak entity set.
this primary key is not formed by the weak entity set completely.
Symbols Used -
-> A double rectangle is used for representing a weak entity set.
-> A double diamond symbol is used for representing the relationship that exists between the strong and weak entity sets and this relationship is known as identifying relationship.
-> A double line is used for representing the connection of the weak entity set with the relationship set.
-> Total participation always exists in the identifying relationship.
Difference Between Strong & Weak Entity Set-
|
Strong entity set
|
Weak entity set
|
|
A single
rectangle is used for the representation of a strong entity set.
|
A double
rectangle is used for
the representation of a weak
entity set.
|
|
It contains sufficient
attributes to form its primary key.
|
It does not
contain sufficient attributes
to form its
primary key.
|
|
A diamond symbol
is used for the representation of the relationship that exists between the
two strong entity sets.
|
A double diamond
symbol is used
for the representation of the identifying
relationship that exists between the strong
and weak entity set.
|
|
A single line is
used for the representation of the connection between the strong entity set
and the relationship.
|
A double
line is used for the
representation of
the connection between
the weak entity
set and the relationship set.
|
|
Total
participation may or may not exist in the relationship.
|
Total
participation always exists in
the identifying
relationship.
|
