The hierarchical data is an example of the composite design pattern. The entity relationship diagrams (aka ERdiagram) are used to represent logical and physical relationships between the database tables. The diagram below shows how the table can be designed to store tree data by maintaining the adjacency information via superior_emp_id.
As you can see the “superior_emp_id” is a foreign key that points to the emp_id in the same table. So, Peter has null as he has no superiors. John and Amanda points to Peter who is their manager or superior and so on.
The above table can be created using SQL DDL (Data Definition Language) as shown below.
CREATE TABLE employee
emp_id NUMBER (4) CONSTRAINT emp_pk PRIMARY KEY,
emp_name VARCHAR2 (40) NOT NULL,
title VARCHAR2 (40),
dept_id NUMBER (2) NOT NULL,
superior_emp_id NUMBER (4) CONSTRAINT emp_fk REFERENCES employee(emp_id)
PRIMARY KEY NONCLUSTERED (emp_id)
This can be represented as an object model to map relational data as shown below
public class Employee
private Long id;
private String name;
private String title;
private Employee superior;
private Set subordinates;
//getters and setters are omitted