Hibernate 迫切连接和普通连接的区别
相关的介绍和解释在代码中已注释，大家可以参考。
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());}}}总结
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