JPA通过查询联接表 [英] JPA join tables through query

问题描述

我有一个奇怪的问题.我想这样创建数据库: 一个学生可以有很多科目.学生对一门学科有一项评估.所以我有 具有ID，姓名，姓氏和A_I ID的班级学生，例如:

I have strange problem. I want to create database like this: One student can has a lot of subjects. Student has one evaluation for one subject. So I have class student with ID, Name, Surname and A_I id, like:

@Id
@GeneratedValue
long id_student;

在学科课上，我有:

    @Id
    @GeneratedValue
    long id_subject;
    String name;
double graduate

我有第三堂课，名为StudentWithGraduate:

I have third class, named StudentWithGraduate:

@Id
@GeneratedValue
long id;
double evaluation;
@OneToOne
Student student;
@OneToOne
Subject subject;

我认为我可以做得更好，但是我不知道如何做.但这不是主要问题.这样，我写的就可以了，但是我想在查询中做一些连接，例如:

I think I could do it better, but I don't know how. But it isn't a main problem. This, what I write up is working, but I want to do some joins in query like:

Vector<Object[]>  v = (Vector<Object[]>) em.createQuery(
"select p.name, o.graduate from Student s 
left join StudentWithGraduate o on s.id_student=o.student
left join Subject p on p.id_subject=o.subject where
s.surname='"+name+"'").getResultList();

它抛出一个错误: 异常描述:对象比较只能与OneToOneMappings一起使用.其他映射比较必须通过查询键或直接属性级别比较来完成.

And it throw an error: Exception Description: Object comparisons can only be used with OneToOneMappings. Other mapping comparisons must be done through query keys or direct attribute level comparisons.

如何更改此数据库方案或更改该查询?

How can I change this DB scheme or change that query?

对不起，我的英语.

PS.当我进行研究时，我发现了@joinTables，但是我不知道如何使用它.

PS. When I was doing research I found @joinTables, but I don't know how to use it..

推荐答案

您需要

You need a short introduction in JPQL, but I will try to quickly explain some missing parts:

在JPQL查询中，您不不写JOIN条件(即ON表达式)，而是从实体图开始浏览实体图(在下面以StudentWithGraduate实体开头):

In a JPQL query you do not write which are the JOIN conditions (i.e the ON expresions), and instead you navigate through your Entity Graph, beginning with an entity (below I begin with the StudentWithGraduate entity):

SELECT p.name, o.graduate FROM StudentWithGraduate o
    LEFT JOIN o.student s
    LEFT JOIN o.subject p
    WHERE s.surname=:name

:name"被称为命名参数，它可以帮助您 SQL注入.为了为其设置一个值，您可以编写以下内容:

The ":name" is called a named parameter, which helps you agains SQL injections. In order to set a value for it, you write the following:

Query query = em.createQuery(aboveQuery);
query.setParameter("name", parameterValue);
//....the rest of parameters + getResultList();

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