Hibernate迫切连接和普通连接的区别实例详解

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());		}	}}