Hibernate Query Language

Hibernate Query Language (HQL) is an object-oriented query language similar to SQL, but designed for Hibernate. It operates on entity objects instead of database tables.

Select All Records:

Equivalent to: SELECT * FROM student;
Uses entity name (Student) instead of table name.

import java.util.List;

Session session = HibernateUtil.getSession();

Transaction transaction = session.beginTransaction();

String hql = "FROM Student";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

// Loop using for-each

for (Student student : students) {

System.out.println("Name: " + student.getName());

System.out.println("----------------------");

}

transaction.commit();

session.close();

Select Specific Columns:

Returns specific fields instead of full entity objects.

import java.util.List;

String hql = "SELECT s.name, s.email FROM Student s";

List<Object[]> results = session.createQuery(hql,Object[].class).getResultList();

for (Object[] row : results) {

System.out.println("Name: " + row[0] + ", email: " + row[1]);

}

transaction.commit();

session.close();

HQL with WHERE Clause:

Uses :parameter to prevent SQL injection.
Equivalent SQL: SELECT * FROM student WHERE name = 'Alice';

Session session = HibernateUtil.getSession();

Transaction transaction = session.beginTransaction();

String hql = "FROM Student WHERE name = :studentName";

Student student = session.createQuery(hql, Student.class)

.setParameter("studentName", "David")

.uniqueResult();

System.out.println(student.getEmail());

transaction.commit();

session.close();

Ordering Results:

ORDER BY

Sorts results alphabetically by name.

String hql = "FROM Student ORDER BY name ASC";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

ORDER BY DESC

Sorts by age in descending order.

String hql = "FROM Student ORDER BY age DESC";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

Filtering with LIKE (Pattern Matching):

Fetches all students whose names start with 'A'.

String hql = "FROM Student WHERE name LIKE 'A%'";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

HQL Joins:

Inner Join

Fetches students enrolled in Java course.

String hql = "SELECT s FROM Student s INNER JOIN s.courses c WHERE c.title = 'Java'";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

Left Join

Includes students even if they don’t have courses.

String hql = "SELECT s FROM Student s LEFT JOIN s.courses c";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

Fetch Join (Eager Loading)

Prevents LazyInitializationException by eagerly fetching related entities.

String hql = "FROM Student s JOIN FETCH s.courses";

List<Student> students = session.createQuery(hql, Student.class).getResultList();

Aggregation Functions (COUNT, SUM, AVG, MIN, MAX):

Count Total Students

String hql = "SELECT COUNT(s) FROM Student s";

Long count = session.createQuery(hql, Long.class).getSingleResult();

Find Sum Age

String hql = "SELECT SUM(s) FROM Student s";

Integer count = session.createQuery(hql, Integer.class).getSingleResult();

Find Average Age

String hql = "SELECT AVG(s.age) FROM Student s";

Double avgAge = session.createQuery(hql, Double.class).getSingleResult();

Find Minimum Age

String hql = "SELECT MIN(s.age) FROM Student s";

Integer minAge = session.createQuery(hql, Integer.class).getSingleResult();

Find Maximum Age

String hql = "SELECT MAX(s.age) FROM Student s";

Integer minAge = session.createQuery(hql, Integer.class).getSingleResult();

Updating Data in HQL:

Equivalent SQL: UPDATE student SET age = 25 WHERE name = 'Alice';

String hql = "UPDATE Student SET age = :newAge WHERE name = :studentName";

int updatedCount = session.createQuery(hql)

.setParameter("newAge", 25)

.setParameter("studentName", "Alice")

.executeUpdate();

Deleting Data in HQL:

Equivalent SQL: DELETE FROM student WHERE name = 'Bob';

String hql = "DELETE FROM Student WHERE name = :studentName";

int deletedCount = session.createQuery(hql)

.setParameter("studentName", "Bob")

.executeUpdate();

Pagination in HQL:

Hibernate allows pagination using setFirstResult() and setMaxResults().

String hql = "FROM Student";

List<Student> students = session.createQuery(hql, Student.class)

.setFirstResult(5) // Skips first 5 records (Page 2)

.setMaxResults(5) // Fetch next 5 records

.getResultList();

Named Queries (@NamedQuery):

A Named Query allows you to define HQL queries inside entity classes.
Benefits: Code is cleaner & easier to maintain.

Defining Named Query:

@Entity

@NamedQuery(name = "Student.findByName", query = "FROM Student WHERE name = :name")

public class Student {

@Id

@GeneratedValue(strategy = GenerationType.IDENTITY)

private Long id;

private String name;

}

Using Named Query:

List<Student> students = session.createNamedQuery("Student.findByName", Student.class)

.setParameter("name", "Alice")

.getResultList();

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