Hibernate 迫切连接和普通连接的区别

package com.baidu.test;

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;
import org.hibernate.service.ServiceRegistryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.baidu.leftJoin.Department;
import com.baidu.leftJoin.Employee;

public class TestHQL_LeftJoin {
    
    private SessionFactory sessionFactory;
    private Session session;
    private Transaction transaction;
    
    
    @Before
    public void init(){
        Configuration configuration = new Configuration().configure();
        ServiceRegistry serviceRegistry = new ServiceRegistryBuilder()
                                        .applySettings(configuration.getProperties())
                                        .buildServiceRegistry();
        
        sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        
        session = sessionFactory.openSession();
        transaction = session.beginTransaction();
    }
    @After
    public void destroy(){
        transaction.commit();
        session.close();
        sessionFactory.close();
        
    }
    
//  ~~~~~~~~~~~~~~~~~~~~~~~~~~下面的例子是 从 1 对  多   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    /**
     *
     * 迫切左外连接: 特点是：如果左表有不满足条件的，也返回左表不满足条件
     *        1. LEFT JOIN FETCH 关键字表示迫切左外连接检索策略.
     *        2. list() 方法返回的集合中存放实体对象的引用, 每个 Department 对象关联的 Employee 集合都被初始化,
     *             存放所有关联的 Employee 的实体对象.
     *        3. 查询结果中可能会包含重复元素, 可以通过一个 HashSet 来过滤重复元素
     *
     *         去重：
     *             方法一:使用 distinct
     *                 String hql  = "SELECT DISTINCT d FROM  Department d LEFT JOIN FETCH d.emps ";
     *                Query query = session.createQuery(hql);
     *
     *                List<Department> depts = query.list();
     *                System.out.println(depts.size());
     *             
     *             方法二
     *                  String hql  = "FROM  Department d LEFT JOIN FETCH d.emps ";
     *                Query query = session.createQuery(hql);
     *
     *                List<Department> depts = query.list();
     *
     *                depts = new ArrayList<>(new LinkedHashSet(depts));
     *                System.out.println(depts.size());
     *                
     *                for(Department dept:depts){
     *                    System.out.println(dept.getName() + "--" + dept.getEmps().size() );
     *                }
     *
     *
     */
    @Test
    public void testLeftJoinFetch(){
//        String hql  = "SELECT DISTINCT d FROM  Department d LEFT JOIN FETCH d.emps ";
//        Query query = session.createQuery(hql);
//        
//        List<Department> depts = query.list();
//        System.out.println(depts.size());
//        
        
        
        String hql  = "FROM  Department d LEFT JOIN FETCH d.emps ";
        Query query = session.createQuery(hql);
        
        
        List<Department> depts = query.list();
        System.out.println(depts.size());
        
        depts = new ArrayList<>(new LinkedHashSet(depts));
        System.out.println(depts.size());
        
        for(Department dept:depts){
            System.out.println(dept.getName() + "--" + dept.getEmps().size() );
        }
    }
    
    
    /**
     * 左外连接:
     *        1. LEFT JOIN 关键字表示左外连接查询.
     *        2. list() 方法返回的集合中存放的是对象数组类型
     *        3. 根据配置文件来决定 Employee 集合的检索策略.
     *        4. 如果希望 list() 方法返回的集合中仅包含 Department 对象,
     *            可以在HQL 查询语句中使用 SELECT 关键字
     *        
     *        这样的语句查询的结果有重复：
     *            String hql = "FROM Department d LEFT JOIN d.emps";
     *            Query query = session.createQuery(hql);
     *        
     *            List<Object[]> results = query.list();
     *            System.out.println(results.size());
     *    
     *         去重：
     *             仅能使用  distinct 的方法去除重复
     *     
     *             String hql = "SELECT DISTINCT d FROM Department d LEFT JOIN d.emps";
     *             Query query = session.createQuery(hql);
     *
     *             List<Department> depts = query.list();
     *             System.out.println(depts.size());
     *                 
     *             for(Department dept:depts){
     *                 System.out.println(dept.getName() + dept.getEmps().size());
     *             }
     *
     */
    @Test
    public void testLeftJoin(){
        String hql = "SELECT DISTINCT d FROM Department d LEFT JOIN d.emps";
        Query query = session.createQuery(hql);
        
        List<Department> depts = query.list();
        System.out.println(depts.size());
        
        for(Department dept:depts){
            System.out.println(dept.getName() + dept.getEmps().size());
        }        
        
    }
    
    /**
     * 迫切内连接: 特点是：不返回左表不满足条件
     *        INNER JOIN FETCH 关键字表示迫切内连接, 也可以省略 INNER 关键字
     *        list() 方法返回的集合中存放 Department 对象的引用, 每个 Department
     *                对象的 Employee 集合都被初始化, 存放所有关联的 Employee 对象
     *
     * 内连接:
     *        INNER JOIN 关键字表示内连接, 也可以省略 INNER 关键字
     *        list() 方法的集合中存放的每个元素对应查询结果的一条记录, 每个元素都是对象数组类型
     *        如果希望 list() 方法的返回的集合仅包含 Department  对象, 可以在 HQL 查询语句中使用 SELECT 关键字
     *
     *
     *
     */
    @Test
    public void testInnerJoinFetch(){
        //String hql  = "SELECT DISTINCT d FROM  Department d LEFT JOIN FETCH d.emps ";
        String hql  = "FROM  Department d INNER JOIN FETCH  d.emps ";
        Query query = session.createQuery(hql);
        
        
        List<Department> depts = query.list();
        depts = new ArrayList<>(new LinkedHashSet(depts));
        System.out.println(depts.size());
        
        for(Department dept:depts){
            System.out.println(dept.getName() + "--" + dept.getEmps().size() );
        }
    }
    
    
//  ~~~~~~~~~~~~~~~~~~~~~~~~~~下面的例子是 从多 对  1   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    
    @Test
    public void testLeftJoinFetch2(){
        String hql = "FROM Employee e LEFT JOIN FETCH e.dept";
        Query query = session.createQuery(hql);
        
        List<Employee> emps = query.list();
        System.out.println(emps.size());
        
        for(Employee emp:emps){
            System.out.println(emp + " -- " + emp.getDept());
        }
        
    }
}