Question

I’m new to EJB and trying to get my head around translating SQL concepts to EJB entity beans.

Suppose I have two tables: PEOPLE (id, name), CONTACT(pid, phone_number). If I want to get a list of all people whether or not they have phone #s, in my EJB session bean I simply issue a SQL query via JDBC such as:

SELECT PEOPLE.name, CONTACT.phone_number 
FROM PEOPLE 
LEFT JOIN CONTACT ON PEOPLE.id = CONTACT.pid

Instead of using SQL/JDBC, I now want to use EJB entity beans. So I create corresponding EJB3 entity bean classes for my tables.

So I now have access to both entity classes from my session bean and I no longer wish to access my database tables directly via SQL/JDBC from my session bean. I only want to use my entity beans and capabilities of JPA. What’s the proper EJB design so that in my session bean, I get the same results as my SQL query?

I’m unclear about how to use the EJB entity bean classes to produce the same results as my SQL outer join query. Help.

Answer 1

First, create two JPA entities, something like that for People:

@Entity
@Table( name="PEOPLE" )
public class People {

    @Id @Column
    private Long id;

    @Column
    private String name;

    @OneToOne
    @JoinColumn( name="pid" )
    private Contact contact;

    // getters and setters
}

And for Contact:

@Entity
@Table( name="CONTACT" )
public class Contact {

    @Id @Column
    private Long pid;

    @Column
    private String phoneNumber;

    // getters and setters
}

Then, to generate the OUTER JOIN query using JPQL:

SELECT p FROM People p LEFT JOIN p.contact c