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Table of contents
What is Cardinality?
Why is Cardinality Important in Database?
What is Mapping Cardinality?
Types of Cardinality in DBMS
1. One-to-One Cardinality (1:1)
2. One-to-Many Cardinality (1)
3. Many-to-One Cardinality (m:1)
4. Many-to-Many Cardinality (m)
Appropriate Mapping Cardinality
Advantages of Cardinality in DBMS
Frequently Asked Questions
What are cardinality constraints?
What is degree and cardinality in DBMS?
What is the cardinality of the DBMS formula?
What are the 3 types of cardinality ratios?
Last Updated: Mar 27, 2024

Cardinality in DBMS

Author Muskan Sharma
Master Python: Predicting weather forecasts
Ashwin Goyal
Product Manager @


While working on a database we always came up with some relationships between the entitites these relations are called as cardinality. So in this blog, you'll be introduced to the concept of cardinality in DBMS, explore its purpose, and how it plays a vital role in the performance of the database because of the Query execution plan. The query execution plan is a set of instructions that enables the user to search and access the data in the database. 

Cardinality in DBMS

What is Cardinality?

In Mathematics, cardinality means the number of members in a set. But in DBMS, the definition of cardinality is different. The uniqueness of data present in a column is called cardinality. 

A column with a high cardinality means that a significant percentage of the values are unique. Low cardinality refers to a column with a large number of repeated values.

Now let’s have a look at the concept of mapping cardinality.

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Why is Cardinality Important in Database?

Cardinality is important in the Database as it creates links between different entities inside a table. It has a huge impact on the query execution plan, that is, the number of steps a user takes to search for a particular value/object inside the database.

Cardinality is also critical in optimizing database performance. The database optimizer can select the most efficient query execution plan by precisely assessing the cardinality of distinct characteristics or columns in a table. A greater cardinality indicates more distinct values, leading to improved selectivity and faster query execution. A reduced cardinality, on the other hand, may suggest a limited number of distinct values, allowing the database to use specialized optimization techniques such as index selection and join algorithms.

What is Mapping Cardinality?

Mapping cardinality refers to the relationship between two entities in a database. It specifies how many instances of one entity can be linked to instances of another entity. It aids in understanding the nature and limits of relationships, allowing for appropriate database design and ensuring data integrity throughout Data manipulation procedures.

Mapping Cardinality

Types of Cardinality in DBMS

When one attribute of a particular entity refers to another attribute of another entity, some relationship exists between both entities. How the elements of one entity are related to another entity can be defined using the Mapping Cardinality.
There are four types of Mapping cardinality in a database, which are:

  • One-to-One cardinality (1:1)
  • One-to-Many cardinality (1)
  • Many-to-One cardinality (m:1)
  • Many-to-Many cardinality (m)

Let's discuss each of these in detail in the upcoming sections.

1. One-to-One Cardinality (1:1)

An entity in A is associated with at most one entity in B in this form of cardinality mapping.

One-to-One cardinality

Let's have a look at the example for more clarity. 


One-to-one example

Here, this relationship is one-to-one relationship because one team is led by one team leader.

2. One-to-Many Cardinality (1)

An entity set A is associated with any number of entities in B (including zero), and each entity in B is associated with just one entity in A.

One-to-Many cardinality

Let's have a look at the example for more clarity. 


One-to-many example


Here, this is one-to-many relationship because one team has many team members.

3. Many-to-One Cardinality (m:1)

An entity set in A can only be associated with one entity in B. In contrast, an entity set in B can be associated with any number of entities in A, including zero.

Many-to-One cardinality


Let's have a look at the example for more clarity. 


Many-to-one example

Here, this many-to-one relationship because many employees work in Coding Ninja.

4. Many-to-Many Cardinality (m)

Any entity in A and B is associated with many entities in B and A, respectively, which means many rows in the first table are associated with many rows in the second table.

Many-to-Many cardinality


Consider this example for better understanding.

Many-to-many example


Here, this is many-to-many relationship because many employees works on many projects.

Appropriate Mapping Cardinality

The proper mapping cardinality for a given relation set is obviously determined by the context in which the relation set is modeled.

  • We can mix relational tables with multiple involved tables if the cardinality is one-to-many or many-to-one
  • We can't mix any two tables if the cardinality is many-to-many
  • If we have a one-to-one relation and one entity has complete participation, we can mix that entity with a relation table, and if both entities have complete participation, we can build one table by combining the two entities and their relation

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Advantages of Cardinality in DBMS

  • Cardinality is crucial because it creates organized relationships from one table or entity to another. This has a significant impact on how the query is executed
  • A query execution plan is a set of actions users can use to find and retrieve data in a database system
  • A well-structured query execution plan can help consumers find the information they need more quickly

Frequently Asked Questions

What are cardinality constraints?

Cardinality constraints define the minimum and maximum number of times one entity can relate to another in a database relationship. It's like setting rules for how things connect.

What is degree and cardinality in DBMS?

The degree is the number of entities involved in a particular relationship. It refers to the number of different attributes or columns in a table, while Cardinality is used to count the total number of tuples or rows in a table.

What is the cardinality of the DBMS formula?

The cardinality formula for a DBMS represents the number of different values in a database table column or attribute. It is an important factor in query optimization and performance, as it influences the effectiveness of search operations and query execution strategies.

What are the 3 types of cardinality ratios?

The cardinality ratio for a binary relationship defines the maximum number of times a particular instance of an entity can be related to the instance of another entity. Three types of cardinality ratios are 1:1, 1:N, and N:M, respectively, for a One-to-One, One-to-Many, and Many-to-Many relationship.


In this blog, we have extensively discussed cardinality in DBMS, how the mapping cardinalities work, the cardinality types, and how cardinality impacts the database.

We hope that this blog has helped you learn more about Cardinality in DBMS in depth.

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