JPA - JPQL Join With Single Valued Path Expression

Following example shows that a JOIN query can be executed for a single valued relationship (such as @OneToOne) as well (other than for collection based relationship).

@Entity
public class Employee {
  @Id
  @GeneratedValue
  private long id;
  private String name;
  @OneToOne(cascade = CascadeType.ALL)
  private Task task;
    .............
}

@Entity
public class Task {
  @Id
  @GeneratedValue
  private long id;
  private String description;
  private String supervisor;
    .............
}

public class ExampleMain {
  private static EntityManagerFactory entityManagerFactory =
          Persistence.createEntityManagerFactory("example-unit");

  public static void main(String[] args) {
      try {
          persistEmployees();
          executeQuery();
          executeQuery2();
      } finally {
          entityManagerFactory.close();
      }
  }

  public static void persistEmployees() {
      Task task1 = new Task("Coding", "Denise");
      Task task2 = new Task("Refactoring", "Rose");
      Task task3 = new Task("Designing", "Denise");

      Employee employee1 = Employee.create("Diana", task1);
      Employee employee2 = Employee.create("Mike", task2);
      Employee employee3 = Employee.create("Tim", task3);
      Employee employee4 = Employee.create("Jack", null);

      EntityManager em = entityManagerFactory.createEntityManager();
      em.getTransaction().begin();
      em.persist(employee1);
      em.persist(employee2);
      em.persist(employee3);
      em.persist(employee4);
      em.getTransaction().commit();
      em.close();
      System.out.println("-- Employee persisted --");
      System.out.println(employee1);
      System.out.println(employee2);
      System.out.println(employee3);
      System.out.println(employee4);
  }

  private static void executeQuery() {
      System.out.println("-- executing JOIN query --");
      EntityManager em = entityManagerFactory.createEntityManager();
      Query query = em.createQuery("SELECT DISTINCT e FROM Employee e INNER JOIN e.task t");
      List<Employee> resultList = query.getResultList();
      for (Employee employee : resultList) {
          System.out.println(employee.getName() + " - " + employee.getTask());
      }
      em.close();
  }

  private static void executeQuery2() {
      System.out.println("-- executing JOIN query with WHERE --");
      EntityManager em = entityManagerFactory.createEntityManager();
      Query query = em.createQuery("SELECT DISTINCT e FROM Employee e INNER JOIN e.task t "
              + "where t.supervisor='Denise'");
      List<Employee> resultList = query.getResultList();
      for (Employee employee : resultList) {
          System.out.println(employee.getName() + " - " + employee.getTask());
      }
      em.close();
  }
}

-- Employee persisted --
Employee{id=1, name='Diana', task=Task{id=2, description='Coding', supervisor='Denise'}}
Employee{id=3, name='Mike', task=Task{id=4, description='Refactoring', supervisor='Rose'}}
Employee{id=5, name='Tim', task=Task{id=6, description='Designing', supervisor='Denise'}}
Employee{id=7, name='Jack', task=null}
-- executing JOIN query --
Mike - Task{id=4, description='Refactoring', supervisor='Rose'}
Tim - Task{id=6, description='Designing', supervisor='Denise'}
Diana - Task{id=2, description='Coding', supervisor='Denise'}
-- executing JOIN query with WHERE --
Tim - Task{id=6, description='Designing', supervisor='Denise'}
Diana - Task{id=2, description='Coding', supervisor='Denise'}

Example Project

Dependencies and Technologies Used:

• h2 1.4.197: H2 Database Engine.
• hibernate-core 5.2.13.Final: The core O/RM functionality as provided by Hibernate.
  Implements javax.persistence:javax.persistence-api version 2.1
• JDK 1.8
• Maven 3.3.9