Tag: er diagrams tutorial

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

 

Example-

 

Consider the following Student table-

 

Roll_noNameAge
1Akshay20
2Rahul19
3Pooja20
4Aarti19

 

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-

 

 

Here,

  • 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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