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 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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