Tag: ER diagrams

ER Diagrams | ER Diagram Symbols

ER diagram-


  • ER diagram or Entity Relationship diagram is a conceptual model that gives the graphical representation of the logical structure of the database.
  • It shows all the constraints and relationships that exist among the different components.


Components of ER diagram-


An ER diagram is mainly composed of following three components-

  1. Entity Sets
  2. Attributes
  3. Relationship Set




Consider the following Student table-


Roll_no Name Age
1 Akshay 20
2 Rahul 19
3 Pooja 20
4 Aarti 19


This complete table is referred to as “Student Entity Set” and each row represents an “entity”.


Representation as ER Diagram-


The above table may be represented as ER diagram as-




  • Roll_no is a primary key that can identify each entity uniquely.
  • Thus, by using student’s roll number, a student can be identified uniquely.


ER Diagram Symbols-


An ER diagram is composed of several components and each component in ER diagram is represented using a specific symbol.

ER diagram symbols are discussed below-


1. For 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 or a company.


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 possess its own primary key.
  • It is represented using a single rectangle.


2. Weak Entity Set-


  • A weak entity set do not possess its own primary key.
  • It is represented using a double rectangle.



Read more- Entity Sets in DBMS


2. For Relationship Sets-


  • Relationship defines an association among several entities.
  • A relationship set is a set of same type of relationships.


A relationship set may be of the following two types-



  1. Strong relationship set
  2. Weak relationship set


1. Strong Relationship Set-


  • A strong relationship exists between two strong entity sets.
  • It is represented using a diamond symbol.


2. Weak Relationship Set-


  • A weak or identifying relationship exists between the strong and weak entity set.
  • It is represented using a double diamond symbol.



Read more- Relationship Sets in DBMS


3. For Attributes-


  • Attributes are the properties which describes the entities of an entity set.
  • There are several types of attributes.



Read more- Attributes in ER Diagram


4. For Participation Constraints-


Participation constraint defines the least number of relationship instances in which an entity has to necessarily participate.

There are two types of participation constraints-



  1. Partial participation
  2. Total participation


1. Partial Participation-


Partial participation is represented using a single line between the entity set and relationship set.


2. Total Participation-


Total participation is represented using a double line between the entity set and relationship set.



Read more- Participation Constraints in DBMS


5. For Specialization and Generalization-


  • Generalization is a process of forming a generalized super class by extracting the common characteristics from two or more classes.
  • Specialization is a reverse process of generalization where a super class is divided into sub classes by assigning the specific characteristics of sub classes to them.



6. For Cardinality Constraints / Ratios-


Cardinality constraint defines the maximum number of relationship instances in which an entity can participate.

There are 4 types of cardinality ratios-



  1. Many-to-many cardinality (m:n)
  2. Many-to-one cardinality (m:1)
  3. One-to-many cardinality (1:n)
  4. One-to-one cardinality (1:1)



Read more- Cardinality Ratios in DBMS

Next Article- Entity Sets in DBMS


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