Types of Relationships- Cardinality

In terms of data models, cardinality refers to the relationship between two tables.  
Relationship can be of four types 

• One-to-One Relationships
• One-to-Many Relationships 
• May to One Relationships 
• Many-to-Many Relationships

1. One-to-one:

One entity from entity set X can be associated with at most one entity of entity set Y and vice versa. A single row of first table associates with single row of second table. 

For example, 

Here,

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

2. One-to-many:

One entity from entity set X can be associated with multiple entities of entity set Y, but an entity from entity set Y can be associated with at least one entity. A single row of first table associates with more than one rows of second table. 

For example,

Here,

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

3. Many to One

More than one entity from entity set X can be associated with at most one entity of entity set Y. However, an entity from entity set Y may or may not be associated with more than one entity from entity set X. Many rows of first table associate with a single row of second table. 

For example,

Here,

• One student can enroll in at most one course.

• One course can be enrolled by any number (zero or more) of students.

4. Many to Many:

One entity from X can be associated with more than one entity from Y and vice versa. Many rows of first table associate with many rows of second table. 

For example,

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.