JPA: JOIN in JPQL | 易学教程

问题

I thought I know how to use JOIN in JPQL but apparently not. Can anyone help me?

select b.fname, b.lname from Users b JOIN Groups c where c.groupName = :groupName

This give me Exception

org.eclipse.persistence.exceptions.JPQLException
Exception Description: Syntax error parsing the query
Internal Exception: org.eclipse.persistence.internal.libraries.antlr.runtime.EarlyExitException

Users have a OneToMany relationship with Groups.

Users.java

@Entity
public class Users implements Serializable{

    @OneToMany(mappedBy="user", cascade=CascadeType.ALL)
    List<Groups> groups = null;
}

Groups.java

@Entity
public class Groups implements Serializable {
    @ManyToOne
    @JoinColumn(name="USERID")
    private Users user;
}

My second question is let say this query return a unique result, then if I do

String temp = (String) em.createNamedQuery("***")
    .setParameter("groupName", groupName)
    .getSingleResult();

*** represent the query name above. So does fname and lname concatenated together inside temp or I get a List<String> back?

回答1:

Join on one-to-many relation in JPQL looks as follows:

select b.fname, b.lname from Users b JOIN b.groups c where c.groupName = :groupName

When several properties are specified in select clause, result is returned as Object[]:

Object[] temp = (Object[]) em.createNamedQuery("...")
    .setParameter("groupName", groupName)
    .getSingleResult(); 
String fname = (String) temp[0];
String lname = (String) temp[1];

By the way, why your entities are named in plural form, it's confusing. If you want to have table names in plural, you may use @Table to specify the table name for the entity explicitly, so it doesn't interfere with reserved words:

@Entity @Table(name = "Users")     
public class User implements Serializable { ... }

来源：https://stackoverflow.com/questions/3730625/jpa-join-in-jpql

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