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 <span style="color:transparent">来源gaodai#ma#com搞*!代#%^码网</span>= 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 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 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 depts = query.list(); //    System.out.println(depts.size()); // String hql = "FROM Department d LEFT JOIN FETCH d.emps "; Query query = session.createQuery(hql); List 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 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 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 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 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 emps = query.list(); System.out.println(emps.size()); for (Employee emp:emps){ System.out.println(emp + " -- " + emp.getDept()); } } }