This example shows how to use IS EMPTY comparison expression which tests whether a collection-valued relationship has been set.
Examples
Entities
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
@ManyToMany(cascade = CascadeType.ALL)
private List<Task> tasks;
.............
}
@Entity
public class Task {
@Id
@GeneratedValue
private long id;
private String description;
private String supervisor;
.............
}
Using IS EMPTY
public class ExampleMain {
private static EntityManagerFactory entityManagerFactory =
Persistence.createEntityManagerFactory("example-unit");
public static void main(String[] args) {
try {
persistEmployees();
findEmployeeWithNoTask();
findEmployeeWithTasks();
findEmployeeWithNoTaskSupervisor();
} 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");
Task task4 = new Task("Documentation", null);
Employee employee1 = Employee.create("Diana", task1, task3);
Employee employee2 = Employee.create("Mike");
Employee employee3 = Employee.create("Tim", task2, task4);
Employee employee4 = Employee.create("Jack");
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 findEmployeeWithNoTask() {
System.out.println("-- Employees with no tasks --");
EntityManager em = entityManagerFactory.createEntityManager();
Query query = em.createQuery("SELECT e FROM Employee e where e.tasks is EMPTY");
List<Employee> resultList = query.getResultList();
resultList.forEach(System.out::println);
em.close();
}
private static void findEmployeeWithTasks() {
System.out.println("-- Employees with tasks --");
EntityManager em = entityManagerFactory.createEntityManager();
Query query = em.createQuery("SELECT e FROM Employee e where e.tasks is NOT EMPTY");
List<Employee> resultList = query.getResultList();
resultList.forEach(System.out::println);
em.close();
}
private static void findEmployeeWithNoTaskSupervisor() {
System.out.println("-- Employees with no task supervisor --");
EntityManager em = entityManagerFactory.createEntityManager();
//IS EMPTY is used for collections only. To find employee with task having no supervisor use a join
Query query = em.createQuery("SELECT DISTINCT e FROM Employee e INNER JOIN e.tasks t "
+ "where t.supervisor is NULL");
List<Employee> resultList = query.getResultList();
resultList.forEach(System.out::println);
em.close();
}
} -- Employee persisted -- Employee{id=1, name='Diana', tasks=[Task{id=2, description='Coding', supervisor='Denise'}, Task{id=3, description='Designing', supervisor='Denise'}]} Employee{id=4, name='Mike', tasks=[]} Employee{id=5, name='Tim', tasks=[Task{id=6, description='Refactoring', supervisor='Rose'}, Task{id=7, description='Documentation', supervisor='null'}]} Employee{id=8, name='Jack', tasks=[]} -- Employees with no tasks -- Employee{id=4, name='Mike', tasks=[]} Employee{id=8, name='Jack', tasks=[]} -- Employees with tasks -- Employee{id=1, name='Diana', tasks=[Task{id=2, description='Coding', supervisor='Denise'}, Task{id=3, description='Designing', supervisor='Denise'}]} Employee{id=5, name='Tim', tasks=[Task{id=6, description='Refactoring', supervisor='Rose'}, Task{id=7, description='Documentation', supervisor='null'}]} -- Employees with no task supervisor -- Employee{id=5, name='Tim', tasks=[Task{id=6, description='Refactoring', supervisor='Rose'}, Task{id=7, description='Documentation', supervisor='null'}]}
Example ProjectDependencies 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
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