Hibernate Query Language (HQL) with Examples

            Hibernate Query Language (HQL) is an object-oriented query language that is similar to SQL. However, instead of operating on database tables and columns, HQL works with persistent entities and their properties. HQL is a superset of the Java Persistence Query Language (JPQL). Every JPQL query is a valid HQL query, but not every HQL query is valid JPQL.

HQL has its own syntax and grammar. HQL queries are written as strings, which Hibernate translates into SQL queries before executing them against the database. Hibernate also provides APIs that allow you to execute native SQL queries directly when needed.

HQL keywords such as SELECT, FROM, and WHERE are case-insensitive. However, entity names, property names, and aliases are case-sensitive and must match the names defined in your entity classes.

Advantages:--

• Instead of returning plain data, HQL queries return the results as entity objects or tuples of objects that can be accessed, processed, and manipulated directly in your application. This eliminates the need to manually create and populate objects from the database result set.

• Compared to SQL, HQL provides several advanced features, such as pagination, fetch joins, dynamic fetching, and object-oriented querying, making it easier to work with persistent entities.

Examples:

I have a given one example student entity, contains fields id, name and age as below,

Student.java,

package com.adnblog;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "student")
public class Student{
    
      @Id
      @GeneratedValue(strategy = GenerationType.IDENTITY)
      @Column(name = "student_id")
      private Integer id;
     
      @Column(name = "student_name")
      private String name;
     
      @Column(name="student_age")
      private Integer age;
     
      public Integer getId() {
           return id;
      }
 
      public void setId(Integer id) {
           this.id = id;
      }
 
      public String getName() {
           return name;
      }
 
      public void setName(String name) {
           this.name = name;
      }
 
      public Integer getAge() {
           return age;
      }
 
      public void setAge(Integer age) {
            this.age = age;
      }   
}

• HQL SELECT Query Example:--

The below query is to select the student where id is 121.

Query query = session.createQuery("from Student where id= :id");
query.setParameter("id", "121");
List list = query.list();

Equivalent sql query is,

Select * from student where student_id = 121;

The above query is same as SQL Query but in this we can use entity name as table name and field name as column name.

• HQL UPDATE Query Example:-

The below query is to update the student name where id is 121.

Query query = session.createQuery("update Student set name= :name" +
        " where id= :id");
query.setParameter("name", "Mahesh");
query.setParameter("id", 121);
int result = query.executeUpdate();

Equivalent sql query is,

UPDATE student SET student_name = 'Mahesh'
          WHERE student_id= 121;

• HQL DELETE Query Example:-

The below HQL query is to delete the Student from student table where id is 121.

Query query = session.createQuery("delete Student where id= :id");
query.setParameter("id", 121);
int result = query.executeUpdate();

Equivalent SQL query is,

DELETE FROM student where student_id = 121;

• HQL INSERT Query Example:

          HQL supports only the INSERT INTO .....  SELECT……… ; there is no chance to write
 INSERT INTO ..... VALUES, it means while writing the insert query, we need to select
values from other table, we can’t insert our own values manually.

Query query = session.createQuery("insert into Student(id, name)" +
       "select student_id, student_name from other_student_table");
int result = query.executeUpdate();

Equivalent SQL Query is,

INSERT INTO student(studnet_id, student_name) 

SELECT student_id, student_name FROM other_student_table;

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