使用HQL从数据库获取数据 [英] Getting data from database using HQL

问题描述

你好， 我有3个表的数据库Staff:

Hi, I have database Staff with 3 tables:

mysql> describe person;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| person_id  | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| date       | datetime     | YES  |     | NULL    |                |
| first_name | varchar(255) | YES  |     | NULL    |                |
| last_name  | varchar(255) | YES  |     | NULL    |                |
| position   | varchar(255) | YES  |     | NULL    |                |
| salary     | double       | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

mysql> describe department;
+---------------+--------------+------+-----+---------+----------------+
| Field         | Type         | Null | Key | Default | Extra          |
+---------------+--------------+------+-----+---------+----------------+
| department_id | bigint(20)   | NO   | PRI | NULL    | auto_increment |
| dept_name     | varchar(255) | YES  |     | NULL    |                |
+---------------+--------------+------+-----+---------+----------------+

mysql> describe department_person;
+---------------+------------+------+-----+---------+-------+
| Field         | Type       | Null | Key | Default | Extra |
+---------------+------------+------+-----+---------+-------+
| department_id | bigint(20) | NO   | PRI | NULL    |       |
| person_id     | bigint(20) | NO   | PRI | NULL    |       |
+---------------+------------+------+-----+---------+-------+

因此，我想编写HQL语句，以便使用person.lastName从person.position获取数据，并且我使用以下代码提取了这些数据，但没有运气:

So, I want to compose HQL statement for getting data from person.position using person.lastName and I've used the following code to exctract this data but without luck:

public List<Person> findPosition(){
        Session session =
                HiberUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();
        List<Person> result = session.createQuery("select position from Person p where p.lastName= 'Anderson'").list();
        for(Person a : result) {
            Hibernate.initialize(a.getDepartmentList());
        }
        session.getTransaction().commit();
        return result;

    }

这是向我展示此异常的地方:

In this was it shows me this exception:

Hibernate: select person0_.position as col_0_0_ from person person0_ where person0_.last_name='Anderson'
Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to main.Person
    at main.StaffDAO.findPosition(StaffDAO.java:100)
    at main.Main.main(Main.java:37)

您能帮我撰写正确的HQL声明吗?

Can you please help me in composing correct HQL statement?

推荐答案

List<Person> result = session.createQuery("select position from Person p where p.lastName= 'Anderson'").list();

您正在选择一个包含VARCHAR(255)的属性，但希望它以List<Person>的形式返回.那根本无法计算.

You are selecting an attribute that holds a VARCHAR(255) but expect it returned as a List<Person>. That simply does not compute.

你可以做

List<Person> result = session.createQuery("select p from Person p where p.lastName= 'Anderson'").list();

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