Tag: strong entity

Entity Sets in DBMS

Entity Set in DBMS-

 

Before you go through this article, make sure that you have gone through the previous article on Introduction to ER Diagrams.

 

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.

 

Types of Entity Sets-

 

An entity set may be of the following two types-

 

 

  1. Strong entity set
  2. Weak entity set

 

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.

 

Example-

 

Consider the following ER diagram-

 

 

In this ER diagram,

 

  • Two strong entity sets “Student” and “Course” are related to each other.
  • Student ID and Student name are the attributes of entity set “Student”.
  • Student ID is the primary key using which any student can be identified uniquely.
  • Course ID and Course name are the attributes of entity set “Course”.
  • Course ID is the primary key using which any course can be identified uniquely.
  • Double line between Student and relationship set signifies total participation.
  • It suggests that each student must be enrolled in at least one course.
  • Single line between Course and relationship set signifies partial participation.
  • It suggests that there might exist some courses for which no enrollments are made.

 

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.

 

NOTE-

 

  • 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.
  • Thus, this combination serves as a primary key for the weak entity set.
  • Clearly, 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.

 

Example-

 

Consider the following ER diagram-

 

 

In this ER diagram,

 

  • One strong entity set “Building” and one weak entity set “Apartment” are related to each other.
  • Strong entity set “Building” has building number as its primary key.
  • Door number is the discriminator of the weak entity set “Apartment”.
  • This is because door number alone can not identify an apartment uniquely as there may be several other buildings having the same door number.
  • Double line between Apartment and relationship set signifies total participation.
  • It suggests that each apartment must be present in at least one building.
  • Single line between Building and relationship set signifies partial participation.
  • It suggests that there might exist some buildings which has no apartment.

 

To uniquely identify any apartment,

  • First, building number is required to identify the particular building.
  • Secondly, door number of the apartment is required to uniquely identify the apartment.

 

Thus,

Primary key of Apartment

= Primary key of Building + Its own discriminator

= Building number + Door number

 

Differences between Strong entity set and Weak entity set-

 

Strong entity setWeak 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.

 

Important Note-

 

In ER diagram, weak entity set is always present in total participation with the identifying relationship set.

So, we always have the picture like shown here-

 

 

Next Article- Relationship Sets in DBMS

 

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