Relationships in database management system

Relationships in Database:

A relationship is a bond between the different objects in a database. It works by matching data in key columns usually columns with the same name in both tables.

In most cases, the relationship matches the primary key from one table, which is a unique identifier for each row, with an entry in the foreign key in the other table.

There are three types of relationships between tables:

•       One-to-One Relationship

•       One-to-Many Relationship

•       Many-to-Many Relationship

One-to-One Relationships:

In this type of relationship, a row in table A can have no more than one matching row in table B, and vice versa. A one-to-one relationship is created if both of the related columns are primary keys or have unique constraints.

Example: You can consider Country and Capital City relationship as One-to-One because one country will be associated with only one Capital City and One Capital City will be associated with only one country.

One-to-Many Relationships

In this type of relationship, a row in table A can have many matching rows in table B, but a row in table B can have only one matching row in table It is the most common type of relationship.

Example: You can consider Country and City relationship as One-to-Many because one country will be associated with many cities where as one city will be associated with only one country

Many-to-Many Relationships

In this type of relationship, a row in table A can have many matching rows in table B, and vice versa. You create such a relationship by defining a third table, called a junction table, whose primary key consists of the foreign keys from both table A and table B.

Example: You can consider Country and River as Many-to-Many relationship because one country may have different rivers passing through it, in the same way a river may pass through many countries