Hibernate 迫切連接和普通連接的區別
相關的介紹和解釋在代碼中已註釋，大家可以參考。
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();}// ~~~~~~~~~~~~~~~~~~~~~~~~~~下面的例子是從1 對多~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~/** * * 迫切左外連接: 特點是：如果左表有不滿足條件的，也返回左表不滿足條件* 1. LEFT JOIN FETCH 關鍵字表示迫切左外連接檢索策略. * 2. list() 方法返回的集合中存放實體對象的引用, 每個Department 對象關聯的Employee 集合都被初始化, * 存放所有關聯的Employee 的實體對象. * 3. 查詢結果中可能會包含重複元素, 可以通過一個HashSet 來過濾重複元素* * 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() );}}/** * 左外連接: * 1. LEFT JOIN 關鍵字表示左外連接查詢. * 2. list() 方法返回的集合中存放的是對像數組類型* 3. 根據配置文件來決定Employee 集合的檢索策略. * 4. 如果希望list() 方法返回的集合中僅包含Department 對象, * 可以在HQL 查詢語句中使用SELECT 關鍵字* * 這樣的語句查詢的結果有重複： * 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： * 僅能使用distinct 的方法去除重複* * 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());}}/** * 迫切內連接: 特點是：不返回左表不滿足條件* INNER JOIN FETCH 關鍵字表示迫切內連接, 也可以省略INNER 關鍵字* list() 方法返回的集合中存放Department 對象的引用, 每個Department * 對象的Employee 集合都被初始化, 存放所有關聯的Employee 對象* * 內連接: * INNER JOIN 關鍵字表示內連接, 也可以省略INNER 關鍵字* list() 方法的集合中存放的每個元素對應查詢結果的一條記錄, 每個元素都是對像數組類型* 如果希望list() 方法的返回的集合僅包含Department 對象, 可以在HQL 查詢語句中使用SELECT 關鍵字* * * */@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() );}}// ~~~~~~~~~~~~~~~~~~~~~~~~~~下面的例子是從多對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());}}}總結
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