java 使用 HQL 从它们之间具有一对多关系的连接表中进行选择
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Using HQL to select from joined tables having one to many relationship between them
提问by Arun Stanislavose
I am stuck with this Hibernate thing and dont know how to figure it out. Please help !
我被这个 Hibernate 的东西困住了，不知道如何弄清楚。请帮忙 ！
So I have these two tables:
所以我有这两个表：
POSITION
positionid(PK), description
JOB
jobid(PK),positionid(FK),description
How do I use HQL in Hibernate to fetch all jobs with their corresponding position descriptions ?
如何在 Hibernate 中使用 HQL 来获取所有作业及其相应的职位描述？
Edit: So this what I am trying to achieve:
编辑：所以这就是我想要实现的目标：
JOBID POSITION.DESCRPTION JOB.DESCRIPTION
1 Teacher Science Teacher
2 Coach Football Coach
and so on for all job's in JOB table. I am trying to figure out what will be HQL for this.
等等，适用于 JOB 表中的所有工作。我想弄清楚这将是什么 HQL。
I have put together the following code till now :
到目前为止，我已经整理了以下代码：
position.hbm.xml
职位.hbm.xml
<hibernate-mapping>
<class name="com.XXXX.model.Position" table="POSITION">
<id name="positionID" type="int" column="POSITIONID" >
<generator class="assigned"/>
</id>
<property name="description">
<column name="DESCRIPTION" />
</property>
<set name="jobs">
<key column="positionID" />
<one-to-many class="com.XXXX.model.Job" />
</set>
</class>
</hibernate-mapping>
job.hbm.xml
作业.hbm.xml
<hibernate-mapping>
<class name="com.XXXX.model.Job" table="JOB">
<id name="jobID" type="int" column="JOBID" >
<generator class="assigned"/>
</id>
<property name="description">
<column name="DESCRIPTION" />
</property>
<many-to-one name="position" class="com.XXXX.model.Position" column="positionID" />
</class>
</hibernate-mapping>
Position.java
位置.java
public class Position {
private int positionID;
private String description;
private Set<Job> jobs = new HashSet<Job>();
// Getters and Setters for all the above three follows...
}
Job.java
作业.java
public class Job {
private int jobID;
private String description;
private Position position;
// Getters and Setters for all the above three follows...
}
In my code now I use
在我的代码中，我现在使用
session.createQuery("from Position as p left join p.positionID as pid").list();
I know its not exactly correct and I am getting the follow error:
我知道它不完全正确，并且出现以下错误：
java.lang.NullPointerException
at org.hibernate.hql.ast.HqlSqlWalker.createFromJoinElement(HqlSqlWalker.java:317)
at org.hibernate.hql.antlr.HqlSqlBaseWalker.joinElement(HqlSqlBaseWalker.java:3268)
at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElement(HqlSqlBaseWalker.java:3060)
at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromElementList(HqlSqlBaseWalker.java:2938)
at org.hibernate.hql.antlr.HqlSqlBaseWalker.fromClause(HqlSqlBaseWalker.java:688)
at org.hibernate.hql.antlr.HqlSqlBaseWalker.query(HqlSqlBaseWalker.java:544)
at org.hibernate.hql.antlr.HqlSqlBaseWalker.selectStatement(HqlSqlBaseWalker.java:281)
.........................
...........................
Can someone tell me how to fix this please ?
有人可以告诉我如何解决这个问题吗？
回答by JB Nizet
If you only want these three columns, then the HQL should be
如果你只想要这三列，那么 HQL 应该是
select job.id, position.decription, job.description
from Job job
left join job.position position
This query will return a List<Object[]>and each Object array in the list will contain these three elements.
此查询将返回 aList<Object[]>并且列表中的每个 Object 数组将包含这三个元素。
It would be much more natural however to use this query:
然而，使用这个查询会更自然：
select job from Job job
left join fetch job.position
which would load all the jobs with their position. The query would return a List, and you would be able to access the three information uou want using job.getId(), job.getPosition().getDescription(), and job.getDescription().
这将加载所有工作及其位置。该查询将返回一个列表，你就可以访问uou想用这三个信息job.getId()，job.getPosition().getDescription()和job.getDescription()。
The syntax of HQL is described with examples in the reference documentation, a must-read.
HQL 的语法在参考文档中通过示例进行了描述，必读。
回答by Chiefumpire
I think you can just getJobs() from the Position object
我认为您可以从 Position 对象中获取 getJobs()
