Related Articles
Recursive Relationships in ER diagrams
• Difficulty Level : Medium
• Last Updated : 10 Jun, 2021

Prerequisite – ER Model
A relationship between two entities of a similar entity type is called a recursive relationship. Here the same entity type participates more than once in a relationship type with a different role for each instance. In other words, a relationship has always been between occurrences in two different entities. However, the same entity can participate in the relationship. This is termed a recursive relationship.

Example –
Let us suppose that we have an employee table. A manager supervises a subordinate. Every employee can have a supervisor except the CEO and there can be at most one boss for each employee. One employee may be the boss of more than one employee. Let’s suppose that REPORTS_TO is a recursive relationship on the Employee entity type where each Employee plays two roles.

1. Supervisor
2. Subordinate

Supervisors and subordinates are called “Role Names”. Here the degree of the REPORTS_TO relationship is 1 i.e. a unary relationship.

• The minimum cardinality of the Supervisor entity is ZERO since the lowest level employee may not be a manager for anyone.
• The maximum cardinality of the Supervisor entity is N since an employee can manage many employees.
• Similarly, the Subordinate entity has a minimum cardinality of ZERO to account for the case where CEO can never be a subordinate.
• Its maximum cardinality is ONE since a subordinate employee can have at most one supervisor.

Note – Here none of the participants have total participation since both minimum cardinalities are Zero. Hence, the relationships are connected by a single line instead of a double line in the ER diagram.

To implement a recursive relationship, a foreign key of the employeeâ€™s manager number would be held in each employee record. A Sample table would look something like this:-

```Emp_entity( Emp_no,Emp_Fname, Emp_Lname, Emp_DOB, Emp_NI_Number, Manager_no);

Manager no - (this is the employee no of the employee's manager)```

This article is contributed by Siddhant Bajaj 2. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.