In Criteria API distinct clause can be applied by using following method of CriteriaQuery :
CriteriaQuery<T> distinct(boolean distinct);
Quick example:
CriteriaQuery<String> query = .......
Root<Employee> employee = query.from(Employee.class);
query.select(employee.get(Employee_.name))
.distinct(true);
Example
Entity
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
private double salary;
private String dept;
.............
}
Applying distinct clause
public class ExampleMain {
private static EntityManagerFactory entityManagerFactory =
Persistence.createEntityManagerFactory("example-unit");
public static void main(String[] args) {
try {
persistEmployees();
findDistinctDepts();
findDistinctSalaries();
findDistinctDeptAndSalaries();
} finally {
entityManagerFactory.close();
}
}
public static void persistEmployees() {
Employee employee1 = Employee.create("Diana", 2000, "IT");
Employee employee2 = Employee.create("Rose", 4500, "Admin");
Employee employee3 = Employee.create("Denise", 2000, "Admin");
Employee employee4 = Employee.create("Mike", 4000, "IT");
Employee employee5 = Employee.create("Linda", 4500, "Sales");
EntityManager em = entityManagerFactory.createEntityManager();
em.getTransaction().begin();
em.persist(employee1);
em.persist(employee2);
em.persist(employee3);
em.persist(employee4);
em.persist(employee5);
em.getTransaction().commit();
System.out.println("-- employee persisted --");
CriteriaQuery<Employee> query = em.getCriteriaBuilder()
.createQuery(Employee.class);
query.select(query.from(Employee.class));
em.createQuery(query).getResultList().forEach(System.out::println);
em.close();
}
private static void findDistinctDepts() {
System.out.println("-- Distinct dept --");
EntityManager em = entityManagerFactory.createEntityManager();
CriteriaQuery<String> query = em.getCriteriaBuilder()
.createQuery(String.class);
Root<Employee> employee = query.from(Employee.class);
query.select(employee.get(Employee_.dept))
.distinct(true);
TypedQuery<String> tq = em.createQuery(query);
List<String> resultList = tq.getResultList();
resultList.forEach(System.out::println);
}
private static void findDistinctSalaries() {
System.out.println("-- Distinct salaries order by salary desc --");
EntityManager em = entityManagerFactory.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Double> query = cb.createQuery(Double.class);
Root<Employee> employee = query.from(Employee.class);
query.select(employee.get(Employee_.salary))
.orderBy(cb.desc(employee.get(Employee_.salary)))
.distinct(true);
TypedQuery<Double> tq = em.createQuery(query);
List<Double> resultList = tq.getResultList();
resultList.forEach(System.out::println);
}
private static void findDistinctDeptAndSalaries() {
System.out.println("-- Distinct dept and salaries order by dept asc and salary desc --");
EntityManager em = entityManagerFactory.createEntityManager();
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Object[]> query = cb.createQuery(Object[].class);
Root<Employee> employee = query.from(Employee.class);
query.multiselect(employee.get(Employee_.dept), employee.get(Employee_.salary))
.orderBy(cb.asc(employee.get(Employee_.dept)), cb.desc(employee.get(Employee_.salary)))
.distinct(true);
TypedQuery<Object[]> tq = em.createQuery(query);
List<Object[]> resultList = tq.getResultList();
resultList.forEach(arr -> System.out.println(Arrays.toString(arr)));
}
} -- employee persisted -- Employee{id=1, name='Diana', salary=2000.0, dept='IT'} Employee{id=2, name='Rose', salary=4500.0, dept='Admin'} Employee{id=3, name='Denise', salary=2000.0, dept='Admin'} Employee{id=4, name='Mike', salary=4000.0, dept='IT'} Employee{id=5, name='Linda', salary=4500.0, dept='Sales'} -- Distinct dept -- IT Sales Admin -- Distinct salaries order by salary desc -- 4500.0 4000.0 2000.0 -- Distinct dept and salaries order by dept asc and salary desc -- [Admin, 4500.0] [Admin, 2000.0] [IT, 4000.0] [IT, 2000.0] [Sales, 4500.0]
Example ProjectDependencies and Technologies Used: - h2 1.4.197: H2 Database Engine.
- hibernate-core 5.3.2.Final: Hibernate's core ORM functionality.
Implements javax.persistence:javax.persistence-api version 2.2 - hibernate-jpamodelgen 5.3.2.Final: Annotation Processor to generate JPA 2 static metamodel classes.
- JDK 1.8
- Maven 3.5.4
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