Hibernate Query Language (HQL)

 Hibernate Query Language (HQL) is an object-oriented query language similar to SQL but operates on Hibernate entities instead of database tables. Here's an example of using HQL to query the database.

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Step 1: Entity Class

import jakarta.persistence.*;

@Entity
@Table(name = "employee")
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

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

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

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

    // Getters and setters
    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDepartment() {
        return department;
    }

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

    public double getSalary() {
        return salary;
    }

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

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Step 2: HQL Query Examples

Example 1: Fetch All Employees

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

import java.util.List;

public class HQLExample {
    public static void main(String[] args) {
        SessionFactory sessionFactory = new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();
        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();

        try {
            // HQL Query
            String hql = "FROM Employee";
            List<Employee> employees = session.createQuery(hql, Employee.class).getResultList();

            for (Employee emp : employees) {
                System.out.println("ID: " + emp.getId() + ", Name: " + emp.getName() +
                        ", Department: " + emp.getDepartment() + ", Salary: " + emp.getSalary());
            }

            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) transaction.rollback();
            e.printStackTrace();
        } finally {
            session.close();
            sessionFactory.close();
        }
    }
}

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Example 2: Fetch Employees by Department

String hql = "FROM Employee WHERE department = :dept";
List<Employee> employees = session.createQuery(hql, Employee.class)
    .setParameter("dept", "HR")
    .getResultList();

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Example 3: Fetch Employees with Salary Greater Than a Threshold

String hql = "FROM Employee WHERE salary > :minSalary";
List<Employee> employees = session.createQuery(hql, Employee.class)
    .setParameter("minSalary", 50000.0)
    .getResultList();

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Example 4: Fetch Employees with Multiple Conditions

String hql = "FROM Employee WHERE department = :dept AND salary > :minSalary";
List<Employee> employees = session.createQuery(hql, Employee.class)
    .setParameter("dept", "IT")
    .setParameter("minSalary", 60000.0)
    .getResultList();

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Example 5: Sorting Results

String hql = "FROM Employee ORDER BY name ASC";
List<Employee> employees = session.createQuery(hql, Employee.class).getResultList();

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Example 6: Aggregation (Count, Max, etc.)

// Count the number of employees
String hql = "SELECT COUNT(e) FROM Employee e";
Long count = session.createQuery(hql, Long.class).getSingleResult();
System.out.println("Total Employees: " + count);

// Get maximum salary
hql = "SELECT MAX(e.salary) FROM Employee e";
Double maxSalary = session.createQuery(hql, Double.class).getSingleResult();
System.out.println("Max Salary: " + maxSalary);

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Example 7: Pagination

String hql = "FROM Employee";
List<Employee> employees = session.createQuery(hql, Employee.class)
    .setFirstResult(0) // Start position
    .setMaxResults(10) // Number of records
    .getResultList();