在Hibernate中“一对一或一”映射中的左联接

Question

I have two tables as below:

Student: oneID (primary key), col1, col2, col3, subId, col4
Subject: subID (primary key), col1, col2

Every student will have exactly one or none of the subjects assigned.

So in the case: one subject assigned: subId in Student will have some value which maps to subID in Subject table

and in the case: 0 subject assigned: subId in Student will be null.

With above scenario, I have a left join query as below:

select st.col1, st.col2, su.col1, su.col2 from Student st left join Subject su on st.subId = su.subId where st.oneId = 'abc'

How to write the same exact query in Hibernate ?

select st.col1, st.col2, su.col1, su.col2 from Student st left join st.sub as su where st.oneId = 'abc'

Above query does not work and gives the below error:

"org.hibernate.hql.ast.QuerySyntaxException: Path expected for join!"

What am I missing ?

In my java code I have kept both the tables independent. That is there is no relationship between the tables defined.

Persistence.xml is as below:

<persistence-unit name="myEntityManager" transaction-type="RESOURCE_LOCAL">              
  <provider>org.hibernate.ejb.HibernatePersistence</provider>    
  <class>com.myPackage.Student</class>       
  <class>com.myPackage.Subject</class>   
</persistence-unit> 

POJOs are as below:

Student

@Entity
@Table(name = "STUDENT")
public class Student implements Serializable {

@Id
@Column(name = "ONEID")
private String oneId;

private Subject sub;

//other columns here

public void setSub(final Subject sub) {
    this.sub = sub;
}

@OneToOne(cascade = CascadeType.ALL)
public Subject getSub() {
    return this.sub;
}
} 

Subject

@Entity
@Table(name = "SUBJECT")
public class Subject implements Serializable {

@Id
@Column(name = "SUBID")
private String subId;

//other columns here
} 

Answer 1

As I don't see any association between your models in mapping you can refer to this link for your problem. Or simply add association in your mapping ie in student add many-to-one association for subject and modify your query as

select st.col1, st.col2, su.col1, su.col2 from Student st left outer join st.subject as su where st.oneId = 'abc' and st.subId = su.subId

Answer 2

As mentioned in this hibernate forum thread , it's currently not possible in Hibernate to invoke left join on disassociated entities. There's also an opened Hibernate issue for it.

What you should do instead, is define a one-to-one or one-to-many relationship between Student and Subject ( one-to-one example , many-to-one example ).

Then, try this HQL, assuming the reference in Student to its subjects is named subjects :

select st.col1, st.col2, su.col1, su.col2 from Student st left join st.subjects su where st.oneId = 'abc'