JPQL and outer join

Question

I have this problem with this simple left join.

I have two tables: Employee and Department

Employee has a many-to-one association with Department:

public class Employee {
    @Id
    @Column(name = "ID")
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SEQ_EMPLOYEE")
    private Long id;

    @Column(name = "NAME")
    private String name;

    @Column(name = "SURNAME")
    private String surname;

    @ManyToOne()    
    private Department department;

And department:

@Entity
@Table(name = "DEPARTMENT")
@SequenceGenerator(name = "SEQ_DEPARTMENT", sequenceName = "SEQ_DEPARTMENT", allocationSize = 1)
public class Department {

    @Id
    @Column(name = "ID")
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SEQ_DEPARTMENT")
    private Long id;

    @Column(name = "NAME")
    private String name;

On db

SELECT * FROM EMPLOYEE;
ID      NAME    SURNAME     DEPARTMENT_ID  
1   Massimo Ugues   1
2   Mazi    Ugues   2

SELECT * FROM DEPARTMENT;
ID      ADDRESS     NAME  
1   Via Gaber   Le betulle

So there is one Employee that does not belong to any Department.

Now I need to load the Employee that has id 2, the one with a departmentId = 2 that is not present on Db.

So this is the JPQL query:

select e, department from Employee e left outer join e.department department where e.name = :name and department.id = :id

And this is the relative sql generated:

SELECT t0.ID, t0.NAME, t0.SURNAME, t0.DEPARTMENT_ID, t1.ID, t1.ADDRESS, t1.NAME FROM {oj EMPLOYEE t0 LEFT OUTER JOIN DEPARTMENT t1 ON (t1.ID = t0.DEPARTMENT_ID)} WHERE ((t0.NAME = ?) AND (t1.ID = ?))
    bind => [Mazi, 2]

The problem is not in the join but in the t1.ID = ? . It should be t0.ID = ? where t0 is the employee table.

Any idea how to make it works?

Kind regards. Massimo

Answer 1

The problem is not Hibernate. The problem is your database. If an employee doesn't belong to any department, it should have NULL in the department_id column, and not some unexisting department ID. You should have a foreign key constraint on employee.department_id to make sure such incoherences never happen. That's the C in ACID.

So, my advice is: fix the data:

update employee set department_id = null where department_id not in (select id from department)

Also, note that doing a left join and forcing the department to have a specific ID is the same as doing an inner join.