10.5 객체지향 쿼리 언어

@BeforeEach
void setUp() {
    Team team = new Team("떡잎마을 방범대");
    teamRepository.save(team);

    Team team2 = new Team("데브코스 2기 C반");
    teamRepository.save(team2);

    User user = new User("짱구");
    user.setTeam(team);
    userRepository.save(user);

    User user2 = new User("유리");
    user2.setTeam(team);
    userRepository.save(user2);

    User user3 = new User("치타");
    userRepository.save(user3);
}

조인

이너조인

@Test
void 이너조인(){
    String jpql = "select u from User u join u.team t";

    List<User> resultList = em.createQuery(jpql, User.class)
            .getResultList();

    for (User user : resultList) {
        System.out.println("user = " + user.getUsername() + " team = " + user.getTeam().getName());
    }
}

Hibernate: 
select user_id, team_id, username 
from user u inner join team t 
on u.team_id=t.team_id

user = 짱구 team = 떡잎마을 방범대
user = 유리 team = 떡잎마을 방범대

레프트 조인

@Test
void 레프트조인(){
    String jpql = "select u from User u left join u.team t";

    List<User> resultList = em.createQuery(jpql, User.class)
            .getResultList();

    for (User user : resultList) {
        System.out.println("user = " + user.getUsername() + " team = " + (user.getTeam()!=null? user.getTeam().getName() : "null"));
    }
}

Hibernate: 
select user_id, team_id, username 
from user u left outer join team t 
on u.team_id=t.team_id

user = 짱구 team = 떡잎마을 방범대
user = 유리 team = 떡잎마을 방범대
user = 치타 team = null

서브 쿼리

select m from Member m
where m.age > (select avg(m2.age) from Member m2)

select m from Member m
where (select count(o) from Order o where m = o.member) > 0

서브 쿼리 지원 함수

• [NOT] EXISTS (subquery)

• {ALL|ANY|SOME} (subquery)

• [NOT] IN (subquery)

JPA 서브 쿼리 한계

• FROM 절에서는 사용 못함