JPQL中JoinColumn的查询值

Question

我有两个JPA实体

public class Job {
  @ManyToOne
  @JoinColumn(name = "service")
  public Service service;

  @Column(name = "queue_time")
  public Long queueTime;

  @Column(name = "run_time")
  public Long runTime;
}

public class Service {
  @Id
  @Column(name = "id")
  public Long id;

  @Column(name = "name")
  public String name;

  @Column(name = "host")
  public String host;
}

现在我想用JPQL做一些聚合查询：

SELECT job.service.id, AVG(job.queueTime), AVG(job.runTime) FROM Job job GROUP BY job.service.id

得到的SQL查询（我使用的是MySQL数据库）如下所示：

SELECT t0.id, AVG(t1.queueTime), AVG(t1.runTime) FROM Service t0, Job t1 WHERE (t0.service = t1.id) GROUP BY t0.id

SELECT t1.service, AVG(t1.queueTime), AVG(t1.runTime) FROM Job t1 GROUP BY t1.service

SELECT job.service, AVG(job.queueTime), AVG(job.runTime) FROM Job job GROUP BY job.service

SELECT t0.id, t0.name, t0.host AVG(t1.queueTime), AVG(t1.runTime) FROM Service t0, Job t1 WHERE (t0.service = t1.id) GROUP BY t0.id, t0.name, t0.host

Answer 1

这个问题解决了我的问题：我如何从JPA ManyToOne映射中检索外键而不击中目标表？

我采取了第二个解决方案(b)为FKs使用只读字段）