[java] HQL Hibernate INNER JOIN

How can I write this SQL query in Hibernate? I would like to use Hibernate to create queries, not create the database.

SELECT * FROM Employee e INNER JOIN Team t ON e.Id_team=t.Id_team

I created entity classes in SQLServer2008,

@Entity
@Table(name="EMPLOYEE")
public class Employee
{
    @Id @GeneratedValue
    @Column(name="ID_EMPLOYEE")
    private int id_employee;
    @Column(name="SURNAME")
    private String surname;
    @Column(name="FIRSTNAME")
    private String firstname;
    @Column(name="ID_PROFESSION")
    private int id_profession;
    @Column(name="ID_BOSS")
    private int id_boss;
    @Column(name="HIRED_DATE")
    private Date hired;
    @Column(name="SALARY")
    private double salary;
    @Column(name="SALARY_ADD")
    private double salary_add;
    @Column(name="ID_TEAM")
    private int id_team;
    //setters and getters
}

@Entity
@Table(name="TEAM")
public class Team
{
    @Id @GeneratedValue
    @Column(name="ID_TEAM")
    private int id_team;
    @Column(name="TEAMNAME")
    private String teamname;
    @Column(name="ADDRESS")
    private String address;
    //setters and getters
}

I tried to build working select query in many ways but it still doesn't work.

SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
Session session = sessionFactory.openSession();                 
session.beginTransaction();
String select = "FROM Employee e INNER JOIN Team t ON e.Id_team=t.Id_team";
Query query = session.createQuery(select);
List elist = query.list();
session.getTransaction().commit();
session.close();    

Maybe something is wrong with entities?

This question is related to java hibernate hql

The answer is


Joins can only be used when there is an association between entities. Your Employee entity should not have a field named id_team, of type int, mapped to a column. It should have a ManyToOne association with the Team entity, mapped as a JoinColumn:

@ManyToOne
@JoinColumn(name="ID_TEAM")
private Team team;

Then, the following query will work flawlessly:

select e from Employee e inner join e.team

Which will load all the employees, except those that aren't associated to any team.

The same goes for all the other fields which are a foreign key to some other table mapped as an entity, of course (id_boss, id_profession).

It's time for you to read the Hibernate documentation, because you missed an extremely important part of what it is and how it works.


import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Entity
@Table(name="empTable")
public class Employee implements Serializable{
private static final long serialVersionUID = 1L;
private int id;
private String empName;

List<Address> addList=new ArrayList<Address>();


@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
@Column(name="emp_id")
public int getId() {
    return id;
}
public void setId(int id) {
    this.id = id;
}
public String getEmpName() {
    return empName;
}
public void setEmpName(String empName) {
    this.empName = empName;
}

@OneToMany(mappedBy="employee",cascade=CascadeType.ALL)
public List<Address> getAddList() {
    return addList;
}

public void setAddList(List<Address> addList) {
    this.addList = addList;
}
}

We have two entities Employee and Address with One to Many relationship.

import java.io.Serializable;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name="address")
public class Address implements Serializable{

private static final long serialVersionUID = 1L;

private int address_id;
private String address;
Employee employee;

@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
public int getAddress_id() {
    return address_id;
}
public void setAddress_id(int address_id) {
    this.address_id = address_id;
}
public String getAddress() {
    return address;
}
public void setAddress(String address) {
    this.address = address;
}

@ManyToOne
@JoinColumn(name="emp_id")
public Employee getEmployee() {
    return employee;
}
public void setEmployee(Employee employee) {
    this.employee = employee;
}
}

By this way we can implement inner join between two tables

import java.util.List;
import org.hibernate.Query;
import org.hibernate.Session;

public class Main {

public static void main(String[] args) {
    saveEmployee();

    retrieveEmployee();

}

private static void saveEmployee() {
    Employee employee=new Employee();
    Employee employee1=new Employee();
    Employee employee2=new Employee();
    Employee employee3=new Employee();

    Address address=new Address();
    Address address1=new Address();
    Address address2=new Address();
    Address address3=new Address();

    address.setAddress("1485,Sector 42 b");
    address1.setAddress("1485,Sector 42 c");
    address2.setAddress("1485,Sector 42 d");
    address3.setAddress("1485,Sector 42 a");

    employee.setEmpName("Varun");
    employee1.setEmpName("Krishan");
    employee2.setEmpName("Aasif");
    employee3.setEmpName("Dut");

    address.setEmployee(employee);
    address1.setEmployee(employee1);
    address2.setEmployee(employee2);
    address3.setEmployee(employee3);

    employee.getAddList().add(address);
    employee1.getAddList().add(address1);
    employee2.getAddList().add(address2);
    employee3.getAddList().add(address3);

    Session session=HibernateUtil.getSessionFactory().openSession();

    session.beginTransaction();

    session.save(employee);
    session.save(employee1);
    session.save(employee2);
    session.save(employee3);
    session.getTransaction().commit();
    session.close();
}

private static void retrieveEmployee() {
    try{

    String sqlQuery="select e from Employee e inner join e.addList";

    Session session=HibernateUtil.getSessionFactory().openSession();

    Query query=session.createQuery(sqlQuery);

    List<Employee> list=query.list();

     list.stream().forEach((p)->{System.out.println(p.getEmpName());});     
    session.close();
    }catch(Exception e){
        e.printStackTrace();
    }
}
}

I have used Java 8 for loop for priting the names. Make sure you have jdk 1.8 with tomcat 8. Also add some more records for better understanding.

 public class HibernateUtil {
 private static SessionFactory sessionFactory ;
 static {
    Configuration configuration = new Configuration();

    configuration.addAnnotatedClass(Employee.class);
    configuration.addAnnotatedClass(Address.class);
                  configuration.setProperty("connection.driver_class","com.mysql.jdbc.Driver");
    configuration.setProperty("hibernate.connection.url", "jdbc:mysql://localhost:3306/hibernate");                                
    configuration.setProperty("hibernate.connection.username", "root");     
    configuration.setProperty("hibernate.connection.password", "root");
    configuration.setProperty("dialect", "org.hibernate.dialect.MySQLDialect");
    configuration.setProperty("hibernate.hbm2ddl.auto", "update");
    configuration.setProperty("hibernate.show_sql", "true");
    configuration.setProperty(" hibernate.connection.pool_size", "10");


   // configuration
    StandardServiceRegistryBuilder builder = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties());
    sessionFactory = configuration.buildSessionFactory(builder.build());
 }
public static SessionFactory getSessionFactory() {
    return sessionFactory;
}
} 

You can do it without having to create a real Hibernate mapping. Try this:

SELECT * FROM Employee e, Team t WHERE e.Id_team=t.Id_team

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