Fetch join 페치 조인을 통해 N+1 문제를 해결해 봅시다!

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

    List<User> resultList = em.createQuery(jpql, User.class).getResultList();
    System.out.println("===========================");
    for (User user : resultList) {
        System.out.println("username : " + user.getUsername());
    }
    System.out.println("===========================");
    for (User user : resultList) {
        System.out.println("username = " + user.getUsername());
        Team team = user.getTeam();
        System.out.println(team.getClass());
        System.out.println("user teamName = " + team.getName());
        System.out.println();
    }
}

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

===========================
username : 짱구
username : 유리
username : 치타
username : 코난
===========================
username = 짱구
class com.study.jpql.domain.Team
user teamName = 해바라기반

username = 유리
class com.study.jpql.domain.Team
user teamName = 해바라기반

username = 치타
class com.study.jpql.domain.Team
user teamName = 장미반

username = 코난
class com.study.jpql.domain.Team
user teamName = 어린이 탐정단

컬렉션 페치 조인

@Test
void 팀_조회_페치_조인() {
    String jpql = "select t from Team t join fetch t.users";

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

    for (Team team : resultList) {
        System.out.print("team = " + team.getName());
        System.out.print(" -> ");
        for (User user : team.getUsers()) {
            System.out.print(user.getUsername()+" ");
        }
        System.out.println();
    }
}

Hibernate: 
select t.id, u.id, t.teamname, u.team_id u.username u.team_id, u.id 
from team t inner join user u
on team0_.id=users1_.team_id

team = 해바라기반 -> 짱구 유리 
team = 해바라기반 -> 짱구 유리 
team = 장미반 -> 치타 
team = 어린이 탐정단 -> 코난

• 쿼리에 distinct 키워드 (sql의 distinct)

• 어플리케이션 레벨에서 중복 데이터의 제거 (주소 기반)

Hibernate: 
select distinct t.id, u.id, t.teamname, u.team_id u.username u.team_id, u.id 
from team t inner join user u
on team0_.id=users1_.team_id
team = 해바라기반 -> 짱구 유리 
team = 장미반 -> 치타 
team = 어린이 탐정단 -> 코난