JPA allows usage of subqueries in WHERE or HAVING clauses. Subqueries must be surrounded by parentheses.

Quick examples:

  Query query = em.createQuery("SELECT e FROM Employee e WHERE (SELECT COUNT(p) FROM e.phoneNumbers p) >= 2");
  List<Employee> resultList = query.getResultList();

  Query query = em.createQuery("SELECT e.dept, AVG(e.salary) FROM Employee e GROUP BY e.dept HAVING AVG(e.salary) > "
                      + "(SELECT AVG(e2.salary) FROM Employee e2)");
  List<Object[]> resultList = query.getResultList();

Example

Entity

@Entity
public class Employee {
    @Id
    @GeneratedValue
    private long id;
    private String name;
    private String dept;
    private long salary;
    @ElementCollection
    private List<String> phoneNumbers;
    .............
}

Using Subqueries in WHERE And HAVING clauses

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

    public static void main(String[] args) {
        try {
            persistEmployees();
            findEmployeeByPhoneCount();
            findDeptHavingAboveNetAverage();
            findEmployeeWithLessThanAverageSalary();
        } finally {
            entityManagerFactory.close();
        }
    }

    public static void persistEmployees() {
        Employee employee1 = Employee.create("Diana", "IT", 3000, "111-111-111", "222-111-111");
        Employee employee2 = Employee.create("Rose", "Admin", 2000, "333-111-111", "444-111-111");
        Employee employee3 = Employee.create("Denise", "Admin", 4000, "555-111-111");
        Employee employee4 = Employee.create("Mike", "IT", 3500, "777-111-111");
        Employee employee5 = Employee.create("Linda", "Sales", 2000);
        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("-- Employees persisted --");
        Query query = em.createQuery(
                "SELECT e FROM Employee e");
        List<Employee> resultList = query.getResultList();
        resultList.forEach(System.out::println);
        em.close();
    }

    private static void findEmployeeByPhoneCount() {
        System.out.println("-- Employees who have more than 2 phones --");
        EntityManager em = entityManagerFactory.createEntityManager();
        Query query = em.createQuery(
                "SELECT e FROM Employee e where (SELECT COUNT(p) FROM e.phoneNumbers p) >= 2");
        List<Employee> resultList = query.getResultList();
        resultList.forEach(System.out::println);
        em.close();
    }

    private static void findDeptHavingAboveNetAverage() {
        EntityManager em = entityManagerFactory.createEntityManager();
        System.out.println("-- net average salary --");
        Object singleResult = em.createQuery("SELECT AVG(e.salary) FROM Employee e")
                                .getSingleResult();
        System.out.println(singleResult);

        System.out.println("-- Dept by average salaries --");
        List<Object[]> list = em.createQuery("SELECT e.dept, AVG(e.salary) FROM Employee e GROUP BY e.dept")
                                .getResultList();
        list.forEach(ar -> System.out.println(Arrays.toString(ar)));

        System.out.println("-- Dept having AVG salaries greater than net AVG salary --");
        Query query = em.createQuery(
                "SELECT e.dept, AVG(e.salary) FROM Employee e GROUP BY e.dept HAVING AVG(e.salary) > "
                        + "(SELECT AVG(e2.salary) FROM Employee e2)");
        List<Object[]> resultList = query.getResultList();
        resultList.forEach(r -> System.out.println(Arrays.toString(r)));
        em.close();
    }

    private static void findEmployeeWithLessThanAverageSalary() {
        System.out.println("-- Employees who have less than average salary --");
        EntityManager em = entityManagerFactory.createEntityManager();
        Query query = em.createQuery(
                "SELECT e FROM Employee e where  e.salary < (SELECT AVG(e2.salary) FROM Employee e2)");
        List<Employee> resultList = query.getResultList();
        resultList.forEach(System.out::println);
        em.close();
    }
}

-- Employees persisted --
Employee{id=1, name='Diana', dept='IT', salary=3000, phoneNumbers=[111-111-111, 222-111-111]}
Employee{id=2, name='Rose', dept='Admin', salary=2000, phoneNumbers=[333-111-111, 444-111-111]}
Employee{id=3, name='Denise', dept='Admin', salary=4000, phoneNumbers=[555-111-111]}
Employee{id=4, name='Mike', dept='IT', salary=3500, phoneNumbers=[777-111-111]}
Employee{id=5, name='Linda', dept='Sales', salary=2000, phoneNumbers=[]}
-- Employees who have more than 2 phones --
Employee{id=1, name='Diana', dept='IT', salary=3000, phoneNumbers=[111-111-111, 222-111-111]}
Employee{id=2, name='Rose', dept='Admin', salary=2000, phoneNumbers=[333-111-111, 444-111-111]}
-- net average salary --
2900.0
-- Dept by average salaries --
[IT, 3250.0]
[Sales, 2000.0]
[Admin, 3000.0]
-- Dept having AVG salaries greater than net AVG salary --
[IT, 3250.0]
[Admin, 3000.0]
-- Employees who have less than average salary --
Employee{id=2, name='Rose', dept='Admin', salary=2000, phoneNumbers=[333-111-111, 444-111-111]}
Employee{id=5, name='Linda', dept='Sales', salary=2000, phoneNumbers=[]}

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