JPA：加入JPQL

问题描述

我以为我知道如何在 JPQL 中使用 JOIN ，但显然不是。任何人都可以帮助我吗？

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

这给我一个例外

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

用户与<$有一个OneToMany关系C $ C>组。

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;
}

我的第二个问题是让这个查询返回一个唯一的结果，然后如果我执行

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();

*** 代表上面的查询名称。那么 fname 和 lname 在 temp 或I中连接在一起得到列表< String> 返回？

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

推荐答案

加入一个 - JPQL中的to-many关系如下所示：

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

当在中指定多个属性时，选择子句，结果返回为对象[] ：

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];

顺便说一下，为什么你的实体以复数形式命名，令人困惑。如果您希望将表名复数形式，可以使用 @Table 明确指定实体的表名，这样就不会干扰保留字：

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 { ... }

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