使用 JPQL 从两个表中选择 [英] select from two tables using JPQL

问题描述

我正在使用 JPQL 来检索数据.我可以使用语句获取数据

I'm using JPQL to retrieve data. I can get data using the statement

List persons = null;
persons = em.createQuery("select p.albumName from PhotoAlbum p , Roleuser r 
where r = p.userId and r.userID = 1");

现在我可以使用这个来获取专辑名称:

Now I can get the album names using this:

int i=0;
for (i=0;i<persons.size(); i++)
{   
     System.out.println("Testing n "+ i +" " +  persons.get(0));
}

现在我想获取专辑名称和名为 firstname

Now I want to get the album name and the roleuser's row named firstname

我正在使用查询

persons = em.createQuery("select r.firstName , p.albumName from PhotoAlbum p ,   
Roleuser r where r = p.userId and r.userID = 1").getResultList();

现在我如何在persons.get(0) 返回一个对象时获取firstname 和albumname 行

Now how do I get the rows firstname and albumname as the persons.get(0) is returning a object

通过运行代码:

 for (i=0;i<persons.size(); i++)
    {
        //r = (Roleuser) persons.get(i);
        System.out.println("Testing n "+ i +" " + persons.get(i));
    }

我明白了:

Testing n 0 [Ljava.lang.Object;@4edb4077
INFO: Testing n 1 [Ljava.lang.Object;@1c656d13
INFO: Testing n 2 [Ljava.lang.Object;@46dc08f5
INFO: Testing n 3 [Ljava.lang.Object;@654c0a43

如何映射 persons.get(0) 并获取 firstname 和 albumname?

How do I map the persons.get(0) and get the firstname and albumname?

推荐答案

现在如何在persons.get(0) 返回一个对象时获取firstname 和albumname 行

Now how do get the rows firstname and albumname as the persons.get(0) is returning a object

在 SELECT 子句中具有多个 select_expressions 的查询返回一个 Object[](或 Object[] 的 List代码>).来自 JPA 规范:

Queries with multiple select_expressions in the SELECT clause return an Object[] (or a List of Object[]). From the JPA specification:

指定的查询结果类型通过查询的 SELECT 子句是实体抽象模式类型，一个状态字段类型，结果聚合函数，a的结果施工作业，或一些这些的顺序.

4.8.1 Result Type of the SELECT Clause

The type of the query result specified by the SELECT clause of a query is an entity abstract schema type, a state-field type, the result of an aggregate function, the result of a construction operation, or some sequence of these.

SELECT 子句的结果类型由结果类型定义select_expressions 包含在它.当多个select_expressions 用于SELECT子句，查询的结果是 Object[] 类型，并且此结果中的元素对应于按照他们的顺序SELECT 子句中的规范和in type 到每个结果类型select_expressions.

The result type of the SELECT clause is defined by the the result types of the select_expressions contained in it. When multiple select_expressions are used in the SELECT clause, the result of the query is of type Object[], and the elements in this result correspond in order to the order of their specification in the SELECT clause and in type to the result types of each of the select_expressions.

所以在你的情况下，你可能想要这样的东西:

So in your case, you probably want something like this:

for (i=0;i<persons.size(); i++) {
    //r = (Roleuser) persons.get(i);
    System.out.println("Testing n " + i + " " + persons.get(i)[0] + ", " + 
        persons.get(i)[1]);
}

请注意，通过在 FROM 子句中使用笛卡尔积和在 WHERE 子句中指定连接条件来指定内部连接，不如指定实体关系上的显式连接(使用 [LEFT [OUTER]| INNER ] JOIN 语法).请参阅规范中的整个部分 4.4.5 联接.

Note that specifying an inner join by the use of a cartesian product in the FROM clause and a join condition in the WHERE clause is less typical than specifying an explicit join over entity relationships (using the [LEFT [OUTER] | INNER ] JOIN syntax). See the whole section 4.4.5 Joins in the specification.

• JPA 1.0 规范
  • 第 4.8.1 节SELECT 子句的结果类型"
  • 第 4.8.2 节SELECT 子句中的构造函数表达式"
  • 第 4.4.5 节连接"

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