Steps to Implement the GROUP BY Clause of JPA

import jakarta.persistence.*;

/**
 * @Entity annotation represents an Employee entity in the application.
 */
@Entity
@Table(name = "employees")
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id; // Unique identifier for the employee.

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName; 

    @Column(name = "salary")
    private double salary; 

    @Column(name = "department")
    private String department; 

    // Constructors, getters, and setters
    public Employee() {
    }

    public Employee(String firstName, String lastName, double salary, String department) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.salary = salary;
        this.department = department;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    public String getDepartment() {
        return department;
    }

    public void setDepartment(String department) {
        this.department = department;
    }
}

package model;

import jakarta.persistence.*;

/**
 * Represents an Department entity in the application.
 */

@Entity
@Table(name = "departments")
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id; // Unique identifier for the address.


    @Column(name = "name") //mapping to name column of department table
    private String name;

    public Department() {  // default constructor of the department

    }

    public Long getId() {  
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }
  
    public void setName(String name) {
        this.name = name;
    }
  
  //parameterized constructor with fields of Id and name

    public Department(Long id, String name) {
        this.id = id;
        this.name = name;
    }
}

import jakarta.persistence.EntityManager;
import jakarta.persistence.EntityManagerFactory;
import jakarta.persistence.Persistence;
import jakarta.persistence.criteria.CriteriaBuilder;
import jakarta.persistence.criteria.CriteriaQuery;
import jakarta.persistence.criteria.Join;
import jakarta.persistence.criteria.Root;
import model.Department;
import model.Employees;
import java.util.List;

public class MainApplication 
{
    public static void main(String[] args) 
    {
        // Create an EntityManagerFactory using persistence unit name
        EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("JpaGroupByExample");

        // Create an EntityManager
        EntityManager entityManager = entityManagerFactory.createEntityManager();

        // Begin a transaction
        entityManager.getTransaction().begin();

        // Example data initialization
        Department itDepartment = new Department();
        itDepartment.setName("IT Department");
        entityManager.persist(itDepartment);

        Department salesDepartment = new Department();
        salesDepartment.setName("Sales Department");
        entityManager.persist(salesDepartment);

        Employees employee1 = new Employees();
        employee1.setFirstName("Mahesh");
        employee1.setLastName("Kadambala");
        employee1.setSalary(80000);
        employee1.setDepartment(itDepartment);
        entityManager.persist(employee1);

        Employees employee2 = new Employees();
        employee2.setFirstName("Eswar");
        employee2.setLastName("Betha");
        employee2.setSalary(75000);
        employee2.setDepartment(salesDepartment);
        entityManager.persist(employee2);

        // Commit the transaction
        entityManager.getTransaction().commit();

        // Use Criteria API to group employees by department and calculate total salary
        CriteriaBuilder cb = entityManager.getCriteriaBuilder();
        CriteriaQuery<Object[]> query = cb.createQuery(Object[].class);
        Root<Employees> root = query.from(Employees.class);
        Join<Employees, Department> departmentJoin = root.join("department");

        query.multiselect(
                departmentJoin,
                cb.sum(root.get("salary"))
        );
        query.groupBy(departmentJoin);

        // Execute the query and retrieve results
        List<Object[]> results = entityManager.createQuery(query).getResultList();

        // Print grouped results
        for (Object[] result : results) {
            Department department = (Department) result[0];
            Double totalSalary = (Double) result[1];
            System.out.println("Department: " + department.getName() + ", Total Salary: " + totalSalary);
        }

        // Close the EntityManager and EntityManagerFactory
        entityManager.close();
        entityManagerFactory.close();
    }
}