Entities in databases can have relationships with other entities. These relationships are called relations. In this article we will learn about the different types of relations.
Basically, a relation is a way to connect two entities.
There are three types of relations:
- One to one relations (1:1)
- One to many relations (1:n)
- Many to many relations (n:n)
If there is a unique relation between two entities, we call it a 1:1 relation.
- Earth:Sun / Sun:Earth
- The earth only has one sun.
- The sun only has one earth.
We speak of 1:N relations when there is a relation between one entity and many other entities.
- Student:Course / Course:Student
- One student can have many courses.
- And one course can have many students.
If there are many relations between two entities in both directions, we call it a N:N relation.
Since N:N relations are not unique, they are not valid in databases. Rather they have to be split into 1:N and 1:1 relations by using additional tables.
- Teacher:Student / Student:Teacher
- One teacher can have many students.
- And one student can have many teachers.