将查询从Postgres传递到JPA

我是新手

我正在尝试将此Postgres查询传递给JPA/JPQL

SELECT
    DISTINCT(srv.code) AS Serv_Cod,
    srv.id AS Serv_id,
    srv.description AS Serv_Desc
FROM db.Category AS cat
    join db.Classification AS cla ON cat.id = cla.cat_id
    join db.Service AS srv ON srv.id = cla.srv_id
WHERE cat.id = 10
ORDER BY srv.id;

现在,我想编写相同的查询,我有具有相同名称表的实体。

分类

@Entity
@Table(name = "Classification", schema = "db")
@Audited
public class Classification implements Identifiable<Long> {


    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "srv_id", nullable = true)
    private Service service;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "cat_id", nullable = true)
    private Category category;

    ....

}

服务

@Entity
@Table(name = "Service", schema = "db")
@Audited
public class Service implements Identifiable<Long> {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    private Long id;

    @Column(name = "code", nullable = false)
    private String code;

    @Column(name = "description", nullable = false)
    private String description;

    ....

}

我在看书,但我很困惑。。。

我不知道怎么写 ON 对于 JOIN ,并为列/字段建立DISTINCT。

Long myID = 25L;
this.em.createQuery("SELECT NEW SomeDto(srv.id, srv.code, srv.description)" 
        + " FROM Classification cla"
                    + "JOIN cla·cat_id cat"
                    + "JOIN cla·srv_id srv"
                    + "WHERE cat.id = :id"
        ,BaseObjectDto.class).setParameter("id", myID).getResultList();

感谢您的宝贵帮助。