**Cardinality Constraint-**

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

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

**Types of Cardinality Ratios-**

There are 4 types of cardinality ratios-

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

**Also read-** **Relationship Sets in DBMS** and **Entity Sets in DBMS**

**1. Many-to-Many Cardinality-**

By this cardinality constraint,

- An entity in set A can be associated with any number (zero or more) of entities in set B.
- An entity in set B can be associated with any number (zero or more) of entities in set A.

**Symbol Used-**

**Example-**

Consider the following ER diagram-

Here,

- One student can enroll in any number (zero or more) of courses.
- One course can be enrolled by any number (zero or more) of students.

**2. Many-to-One Cardinality-**

By this cardinality constraint,

- An entity in set A can be associated with at most one entity in set B.
- An entity in set B can be associated with any number (zero or more) of entities in set A.

**Symbol Used-**

**Example-**

Consider the following ER diagram-

Here,

- One student can enroll in at most one course.
- One course can be enrolled by any number (zero or more) of students.

**3. One-to-Many Cardinality-**

By this cardinality constraint,

- An entity in set A can be associated with any number (zero or more) of entities in set B.
- An entity in set B can be associated with at most one entity in set A.

**Symbol Used-**

**Example-**

Consider the following ER diagram-

Here,

- One student can enroll in any number (zero or more) of courses.
- One course can be enrolled by at most one student.

**4. One-to-One Cardinality-**

By this cardinality constraint,

- An entity in set A can be associated with at most one entity in set B.
- An entity in set B can be associated with at most one entity in set A.

**Symbol Used-**

**Example-**

Consider the following ER diagram-

Here,

- One student can enroll in at most one course.
- One course can be enrolled by at most one student.

**Next Article-** **Participation Constraints**

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