如何使用spring data jpa连接从多个实体返回对象？

Question

我有三个实体：EntityA、EntityB 和 EntityC。我需要使用 spring data jpa 从这些实体中将连接查询的值获取到对象列表中。查询是：

select x.id,x.formNo,x.name, z.testScore, y.semester 
   from  EntityA as x left join EntityB as z on x.id = z.a_id 
    inner join EntityC as y on x.c_id = y.id where x.id=1

这些实体是：

实体A：

  @Entity
  public class EntityA {        
    @Id
    @GeneratedValue
    private Integer id;         
    private String name;        
    private String formNo;

    @OneToOne(mappedBy = "a",fetch=FetchType.LAZY, cascade = CascadeType.REMOVE)    
    private EntityB b;

    @ManyToOne
    @JoinColumn(name = "EntityC_id")
    private EntityC c;
}

实体B：

@Entity
public class EntityB {

@Id
@GeneratedValue
private Integer id;     
private double testScore;

@OneToOne
@JoinColumn(name = "EntityA_id")
private EntityA a;  
}

实体C：

@Entity
public class EntityC {
@Id
@GeneratedValue
private Integer id;     
private String semester;

@OneToMany(mappedBy = "c",fetch=FetchType.LAZY, cascade = CascadeType.REMOVE)
private List<EntityA> a;    
}

我试过这样

@Repository
public interface SomeObjectRepository extends JpaRepository<Object, Integer>{   
public final static String FIND_WITH_QUERY 
    = "select x.id,x.formNo,x.name, z.testScore, y.semester 
   from  EntityA as x left join EntityB as z on x.id = z.a_id 
    inner join EntityC as y on x.c_id = y.id where x.id=:id";

    @Query(FIND_WITH_QUERY)
    public List<Object> getObjects(@Param("id") String id);
  }

Answer 1

您只需要认识到 JPQL 是一种与 SQL 不同的语言，并学习它即可。JPQL 从不使用表名和列名。JPQL 连接依赖于实体之间的关联，而不是子句ON。

因此，查询应该简单地是

select x.id,x.formNo,x.name, z.testScore, y.semester
from EntityA x 
left join x.b z
inner join x.c y
where x.id = :id