What Is Relationship In DBMS?

DBMS relationships depend on the set-up and use of the DBMS. They can be one-to-one, or one-to-many, as well as many-to-one or many-to-many relationships.

• One-to-One Relationships

These relationships have only a single value in each direction. A question entered will have only one possible either way. To use a business example, Mr a manages department 01. Both the question 'who manages department 01' and 'what department does Mr a manage' have only one possible answer.

• One-to-Many or Many-to-One Relationships

For example, the department Mr A manages has 10 employees. For the employees the value is single, as they can only work in one department. For the department, the value is many, as there are 10 employees. In a table listing departments and their employees, the relationship between employees and department is a one-to-many relationship.

In a table listing employees and the departments they work for, the relationship between employees and department is many-to-one.

• Many-to-Many Relationships

Relationships that are multi-valued in either direction are so-called many-to-many relationships. For instance, an employee is able to work on more than one project, and a project can have more than one employee currently working on it.

For instance, Mr A, Mrs B and Mr C are working on projects P009, P010 and P011. The question 'Who works on project P009?' and 'Which project is Mr C working on?' both have multiple answers. The relationship is therefore many-to-many.

As a rule, a DBMS will use all relationships in varying combinations at some point or another. A user may, for instance wish to find out which employee from which department is working with which employee on which project.