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HQL

🚀 Step-by-Step Guide: Hibernate Queries (HQL, Criteria API, Native SQL) in H2

Now that we have implemented CRUD operations using Hibernate, let’s explore different ways to query data:
1️⃣ HQL (Hibernate Query Language) – Object-oriented SQL-like queries.
2️⃣ Criteria API – Programmatic approach to building queries.
3️⃣ Native SQL Queries – Direct SQL execution using Hibernate.

🔹 Step 1: Implement HQL Queries

Hibernate Query Language (HQL) allows querying using entity names instead of table names.

📌 Modify StudentDAO.java to add HQL queries:

package com.example.dao;

import com.example.model.Student;
import com.example.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.Transaction;
import java.util.List;

public class StudentDAO {

    // Fetch all students using HQL
    public List<Student> getAllStudentsHQL() {
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            return session.createQuery("FROM Student", Student.class).list();
        }
    }

    // Fetch students by first name using HQL
    public List<Student> getStudentsByFirstName(String firstName) {
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            return session.createQuery("FROM Student WHERE firstName = :firstName", Student.class)
                          .setParameter("firstName", firstName)
                          .list();
        }
    }

    // Update student email using HQL
    public void updateStudentEmail(int id, String newEmail) {
        Transaction transaction = null;
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            transaction = session.beginTransaction();
            session.createQuery("UPDATE Student SET email = :email WHERE id = :id")
                   .setParameter("email", newEmail)
                   .setParameter("id", id)
                   .executeUpdate();
            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) transaction.rollback();
            e.printStackTrace();
        }
    }

    // Delete student using HQL
    public void deleteStudentHQL(int id) {
        Transaction transaction = null;
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            transaction = session.beginTransaction();
            session.createQuery("DELETE FROM Student WHERE id = :id")
                   .setParameter("id", id)
                   .executeUpdate();
            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) transaction.rollback();
            e.printStackTrace();
        }
    }
}

🔹 Step 2: Implement Criteria API Queries

The Criteria API provides a type-safe, programmatic approach to querying.

📌 Modify StudentDAO.java to add Criteria API queries:

import org.hibernate.Criteria;
import org.hibernate.criterion.Restrictions;
import org.hibernate.query.Query;
import java.util.List;

public class StudentDAO {

    // Fetch students by last name using Criteria API
    public List<Student> getStudentsByLastName(String lastName) {
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            Criteria criteria = session.createCriteria(Student.class);
            criteria.add(Restrictions.eq("lastName", lastName));
            return criteria.list();
        }
    }
}

🔹 Step 3: Implement Native SQL Queries

Native SQL queries allow executing raw SQL statements within Hibernate.

📌 Modify StudentDAO.java to add Native SQL queries:

import org.hibernate.query.NativeQuery;

public class StudentDAO {

    // Fetch all students using Native SQL
    public List<Student> getAllStudentsNative() {
        try (Session session = HibernateUtil.getSessionFactory().openSession()) {
            NativeQuery<Student> query = session.createNativeQuery("SELECT * FROM students", Student.class);
            return query.list();
        }
    }
}

🔹 Step 4: Test Queries in Main.java

📌 Modify Main.java to test these queries

package com.example;

import com.example.dao.StudentDAO;
import com.example.model.Student;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        StudentDAO studentDAO = new StudentDAO();

        // Add students
        studentDAO.saveStudent(new Student("Alice", "Brown", "alice@example.com"));
        studentDAO.saveStudent(new Student("Bob", "Smith", "bob@example.com"));

        // Fetch all students using HQL
        List<Student> studentsHQL = studentDAO.getAllStudentsHQL();
        System.out.println("Students using HQL: " + studentsHQL);

        // Fetch by first name using HQL
        List<Student> aliceList = studentDAO.getStudentsByFirstName("Alice");
        System.out.println("Students named Alice: " + aliceList);

        // Update email using HQL
        studentDAO.updateStudentEmail(1, "alice.new@example.com");

        // Delete student using HQL
        studentDAO.deleteStudentHQL(2);

        // Fetch students by last name using Criteria API
        List<Student> studentsCriteria = studentDAO.getStudentsByLastName("Brown");
        System.out.println("Students with last name Brown: " + studentsCriteria);

        // Fetch students using Native SQL
        List<Student> studentsNative = studentDAO.getAllStudentsNative();
        System.out.println("Students using Native SQL: " + studentsNative);
    }
}

✅ Expected Output

Students using HQL: [Student{id=1, firstName='Alice', lastName='Brown', email='alice@example.com'}, 
                      Student{id=2, firstName='Bob', lastName='Smith', email='bob@example.com'}]
Students named Alice: [Student{id=1, firstName='Alice', lastName='Brown', email='alice@example.com'}]
Students with last name Brown: [Student{id=1, firstName='Alice', lastName='Brown', email='alice.new@example.com'}]
Students using Native SQL: [Student{id=1, firstName='Alice', lastName='Brown', email='alice.new@example.com'}]

✅ Summary

🎯 What We Implemented

✔ HQL Queries (FROM Student, UPDATE, DELETE)
✔ Criteria API Queries (Restrictions.eq("lastName", lastName))
✔ Native SQL Queries (SELECT * FROM students)
✔ Tested queries with different methods

🚀 Next Steps

Would you like to move forward with Spring Integration with Hibernate, or do you want to explore Transaction Management in Hibernate first? 😃

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