hibernate入门(三)——HQL查询（Query）

HQL(Hibernate Query Language)是完全面对对象的查询语句，查询功能强大，具备继承、多态和关联等特性。

Query是Hibernate的查询接口，用于从数据存储源查询对象及控制执行查询的过程，Query包装了一个HQL语句。

hibernate5中 org.hibernate.Query 被 org.hibernate.query.Query 替代。

环境配置等详细内容请看：hibernate入门(一)——基本环境搭建

在包orm.test下创建类 QueryTest.java

package orm.test;

import java.util.List;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.query.Query;
import org.junit.Test;

import orm.entity.User;
import orm.util.HibernateUtil;

public class QueryTest {
    @Test   //简单查询
    public void queryUsersTest() {
        SessionFactory sessionFactory = null;
        Session session = null;
        Transaction transaction = null;

        try {
            sessionFactory = HibernateUtil.getSessionFactory();
            session = sessionFactory.getCurrentSession();
            transaction = session.beginTransaction();
            String hql = "from User";
            Query<User> query = session.createQuery(hql, User.class);
            List<User> list = query.getResultList();
            for (User user : list) {
                System.out.println(user);
            }
            transaction.commit();
        } catch (HibernateException e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }
    }

    @Test   //属性查询
    public void queryByTest() {
        SessionFactory sessionFactory = null;
        Session session = null;
        Transaction transaction = null;

        try {
            sessionFactory = HibernateUtil.getSessionFactory();
            session = sessionFactory.getCurrentSession();
            transaction = session.beginTransaction();
            String hql = "from User where username=:username";
            Query<User> query = session.createQuery(hql, User.class);
            query.setParameter("username", "测试");
            List<User> list = query.getResultList();
            for (User user : list) {
                System.out.println(user);
            }
            transaction.commit();
        } catch (HibernateException e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }
    }

    @Test   //实例化查询
    public void queryTest() {
        SessionFactory sessionFactory = null;
        Session session = null;
        Transaction transaction = null;

        try {
            sessionFactory = HibernateUtil.getSessionFactory();
            session = sessionFactory.getCurrentSession();
            transaction = session.beginTransaction();
            String hql = "select new User(u.username,u.age) from User as u";
            Query<User> query = session.createQuery(hql, User.class);
            List<User> list = query.getResultList();
            for (User user : list) {
                System.out.println(user);
            }
            transaction.commit();
        } catch (HibernateException e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }
    }

    @Test   //统计查询
    public void countTest() {
        SessionFactory sessionFactory = null;
        Session session = null;
        Transaction transaction = null;

        try {
            sessionFactory = HibernateUtil.getSessionFactory();
            session = sessionFactory.getCurrentSession();
            transaction = session.beginTransaction();
            String hql = "select count(*) from User";
            Query<Long> query = session.createQuery(hql,Long.class);
            long count = query.uniqueResult();
            System.out.println("总数为："+count);
            transaction.commit();
        } catch (HibernateException e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }
    }

    @Test   //子查询
    public void subQueryTest() {
        SessionFactory sessionFactory = null;
        Session session = null;
        Transaction transaction = null;

        try {
            sessionFactory = HibernateUtil.getSessionFactory();
            session = sessionFactory.getCurrentSession();
            transaction = session.beginTransaction();
            String hql = "from User u where u.age>(select avg(age) from User)";
            Query<User> query = session.createQuery(hql,User.class);
            List<User> list = query.getResultList();
            for (User user : list) {
                System.out.println(user);
            }
            transaction.commit();
        } catch (HibernateException e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        }
    }
}