[JPA] - JPQL 조인

조인

• 내부조인 : select m from Member m join m.team t
• 외부조인 : select m from Member m left join m.team t
• 세타조인 : select count(m) from Member m, Team t where m.username = t.name

            Team team = new Team();
            team.setName("teamA");
            em.persist(team);

            Member member = new Member();
            member.setUsername("member1");
            member.setAge(10);
            em.persist(member);

            member.setTeam(team);

            em.flush();
            em.clear();

            String query = "select m  from Member m left outer join m.team t";
            List<Member> result = em.createQuery(query, Member.class).getResultList();

@Entity
public class Member {
    @Id @GeneratedValue
    private Long id;
    private String username;
    private int age;

    @ManyToOne
    @JoinColumn(name = "TEAM_ID")
    private Team team;

Hibernate: 
    /* select
        m  
    from
        Member m 
    join
        m.team t */ select
            m1_0.id,
            m1_0.age,
            m1_0.TEAM_ID,
            m1_0.username 
        from
            Member m1_0 
        join
            Team t1_0 
                on t1_0.id=m1_0.TEAM_ID
Hibernate: 
    select
        t1_0.id,
        t1_0.name 
    from
        Team t1_0 
    where
        t1_0.id=?

Team과 조인하고 사용하지도 않았는데 Team을 한번 더 조회한다. 왜? Lazy를 안해줬기때문.

ManyToOne OneToOne은 꼭 Lazy설정을 해주도록하자.

@Entity
public class Member {
    @Id @GeneratedValue
    private Long id;
    private String username;
    private int age;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "TEAM_ID")
    private Team team;

조인대상 필터링

• 예) 회원과 팀을 조인하면서, 팀 이름이 A인 팀만 조인

JPQL:

SELECT m, t FROM Member m LEFT JOIN m.team t on t.name = 'A'

SQL:

SELECT m., t. FROM

Member m LEFT JOIN Team t ON m.TEAM_ID=t.id and t.name='A'

연관관계 없는 엔티티 외부조인

• 예) 회원의 이름과 팀의 이름이 같은 대상 외부 조인

JPQL:

SELECT m, t FROM

Member m LEFT JOIN Team t on m.username = t.name

SQL:

SELECT m., t. FROM

Member m LEFT JOIN Team t ON m.username = t.name