조인

• 내부조인
• 외부조인
• 세타조인
• ON절 활용한 조인: 조인 대상을 미리 필터링(JPA 2.1부터 지원)
• 연관관계 없는 엔티티 외부조인(하이버네이트 5.1부터 지원)

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

            Member member = new Member();
            member.setUsername("member1");
            member.setAge(10);
            member.setTeam(team);

            em.persist(member);

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

            String query1 = "select m from Member m join m.team t"; //내부조인
            String query2 = "select m from Member m left join m.team t"; //외부조인
            String query3 = "select m from Member m, Team t where m.username = t.name"; //세타조인
            String query4 = "select m from Member m left join m.team t on t.name = 'teamA'"; //on절
            String query5 = "select m from Member m left join Team t on m.username = t.name"; //연관관계 없는 경우
            List<Member> resultList = em.createQuery(query5, Member.class)
                    .getResultList();

            tx.commit();

• 실행 쿼리(on절): PK키를 비교함

		select
                member0_.Id as id1_0_,
                member0_.age as age2_0_,
                member0_.TEAM_ID as team_id4_0_,
                member0_.username as username3_0_ 
        from
            Member member0_ 
        left outer join
            Team team1_ 
                on member0_.TEAM_ID=team1_.Id 
                and (
                    team1_.name='teamA'
                )

• 실행 쿼리(연관관계 없는 경우): PK키 비교X

		select
                member0_.Id as id1_0_,
                member0_.age as age2_0_,
                member0_.TEAM_ID as team_id4_0_,
                member0_.username as username3_0_ 
        from
            Member member0_ 
        left outer join
            Team team1_ 
                on (
                    member0_.username=team1_.name
                )