一、一對多

以班級Classes和學生Student為例： 回憶sql語句:
//內鏈接,兩種方式效果一樣,查詢的是兩邊都有的數據
SELECT c.*,s.* FROM classes c,student s WHERE s.cid=c.cid;

SELECT c.cname,s.sname FROM classes c INNER JOIN student s ON s.cid=c.cid;
//左外連接，在內鏈接基礎上，左邊表有而右邊表沒有，兩種方式等效；
SELECT c.* ,s.* FROM student s LEFT OUTER JOIN classes c ON s.cid=c.cid;
SELECT c.* ,s.* FROM student s LEFT? JOIN classes c ON s.cid=c.cid;

//右外連接，在內鏈接基礎上，右邊有而左邊無，兩種方式等效；
SELECT c.* ,s.* FROM classes c RIGHT? OUTER JOIN student s ON s.cid=c.cid;
SELECT c.* ,s.* FROM classes c RIGHT? JOIN student s ON s.cid=c.cid; HQL語句： //查詢所有： from Classes c,Student s where c.cid=s.classes.cid； //選擇某些屬性查詢 select c.cname,s.sname from Classes c,Student s where c.cid=s.classes.cid； //選擇某些屬性，封裝為bean查詢； select new cn.itheima03.hibernate.domain.ClassesView(c.cname,s.sname) ?from Classes c,Student s where c.cid=s.classes.cid； //內鏈接查詢，得到的是兩個bean from Classes c inner join c.students s； //內斂鏈接查詢，得到的是Classes對象，對象中包含studet集合 from Classes c inner join fetch c.students s； from Student s inner join fetch s.classes c； select new cn.itheima03.hibernate.domain.ClassesView(c.cname,s.sname)??from Student s inner join? s.classes c ； from Classes c left outer join fetch c.students s； from Student s left outer join fetch s.classes； 示例代碼：

/** * 1.一對多 * sql:select c.*,s.* from classes c,student s where c.cid=s.cid; * hql:from Classes c,Student s where c.cid=s.classes.cid，注意與上句的區別； * 得到的list是object[]，數組中的元素是Classes和Student對象； * */ @Test public?void?testOneToMany_EQ(){ Session???session =?sessionFactory.openSession(); Query query = session.createQuery(?"from Classes c,Student s where c.cid=s.classes.cid"); List?list?= query.list(); System.?out.println(query.list().size()); session.close(); } /** * 2.帶屬性的查詢； * list中裝的是object[]; */ @Test public?void?testOneToMany_EQ_Property(){ Session???session =?sessionFactory.openSession(); Query query = session.createQuery(?"select c.cname,s.sname from Classes c,Student s where c.cid=s.classes.cid"); query.list(); session.close(); } /** * 3.帶屬性查詢，將查詢結果封裝成一個bean； * 得到的list中裝的是classView對象； */ @Test public?void?testOneToMany_EQ_Property_Constructor(){ Session???session =?sessionFactory.openSession(); Query query = session.createQuery(?"select new cn.itheima03.hibernate.domain.ClassesView(c.cname,s.sname) "?+ "from Classes c,Student s where c.cid=s.classes.cid"); List?list?= query.list(); session.close(); } /** * 4.內連接 * 結果與例子1一樣； */ @Test public?void?testOneToMany_InnerJoin_Query(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Classes c inner join c.students s"); Query query = session.createQuery(buffer.toString()); query.list(); session.close(); } /** * 5.迫切內連接1：獲取所有有學生的班級及班級下的學生； * 要想得到的集合中裝的Classes對象，對象中set集合中裝student，可以使用迫切內鏈接。 * */ @Test public?void?testOneToMany_InnerJoin_Fetch_Query_1(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Classes c inner join fetch c.students s"); Query query = session.createQuery(buffer.toString()); List?list?= query.list(); session.close(); } /** * 6.迫切內連接2 * 從學生端出發； */ @Test public?void?testOneToMany_InnerJoin_Fetch_Query_2(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Student s inner join fetch s.classes c"); Query query = session.createQuery(buffer.toString()); query.list(); session.close(); } /** * 7.迫切內連接3：獲取屬性，封裝結果； * select new cn.itheima03.hibernate.domain.ClassView(c.cname,s.sname) * ??from Student s inner join fetch s.classes c； * 上述的?hql語句會報錯，因為from后面想要的結構和select想要的結構是沖突的，所以 如果在from后面加fetch,不能寫select語句，如果加select，不能寫fetch,兩者只能選擇其一 * */ @Test public?void?testOneToMany_InnerJoin_Fetch_Query_Property(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); //下面的寫法不對； //????????buffer.append("select new cn.itheima03.hibernate.domain.ClassView(c.cname,s.sname) " + //?????????????????" from Student s inner join fetch s.classes c"); //不要fetch; buffer.append(?"select new cn.itheima03.hibernate.domain.ClassesView(c.cname,s.sname)? "?+ "? from Student s inner join? s.classes c "?); Query query = session.createQuery(buffer.toString()); List?list?= query.list(); session.close(); } /** * 8.迫切左外連接 * 從班級出發，得到班級對應的學生 */ @Test public?void?testOneToMany_LeftJoin_Fetch(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Classes c left outer join fetch c.students s"); Query query = session.createQuery(buffer.toString()); List<Classes> list =?query.list(); for?(Classes classes : list) { System.?out.println("classes:"?+classes.getCname()); Set<Student> students = classes.getStudents(); for?(Student student : students) { System.?out.println("?????student:"?+student.getSname()); } } session.close(); } /** * 9.迫切左外連接2 * 從學生出發，得到對應的班級 */ @Test public?void?testOneToMany_RightJoin_Fetch(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Student s left outer join fetch s.classes "); Query query = session.createQuery(buffer.toString()); List<Student> list =?query.list(); for?(Student student : list) { System.?out.println("student:"?+student.getSname()); if?(student.getClasses()!=null) { System.?out.println("?????"?+student.getClasses().getCname()); } } session.close(); }

二。多對多 學生Student和課程Course為例: Student里有裝Course的set集合，Course里也有裝Student的set集合； 多對多與一對多操作差不多

/** * 1.得到所有的學生以及其對應的課程 * 從學生端出發 * list裝的是學生； */ @Test public?void?testManyToMany_LeftJoin_Fecth(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Student s left outer join fetch s.courses"); Query query = session.createQuery(buffer.toString()); List?list?= query.list(); session.close(); } /** * 2.得到所有的課程及課程下對應的學生； * list裝的是課程 */ @Test public?void?testManyToMany_LeftJoin_Fecth_2(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Course c left outer join fetch c.students s"); Query query = session.createQuery(buffer.toString()); query.list(); session.close(); } /** * 3.一對多和多對多的結合 * 得到所有班級下的所有學生以及所有學生下的所有課程； * 從班級出發 */ @Test public?void?testManyToManyAndOneToMany(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Classes c left outer join fetch"?+ " c.students s left outer join fetch s.courses"); Query query = session.createQuery(buffer.toString()); List<Classes> classeList =?query.list(); //去掉集合中的重復元素 Set<Classes> sets =?new?HashSet<Classes>(classeList); classeList =?new?ArrayList<Classes>(sets); System.?out.println(classeList.size()); for(Classes classes:classeList){//遍歷班級 System.?out.println(classes.getCname()); Set<Student> students = classes.getStudents();//得到班級下的學生 for(Student student:students){//遍歷學生 System.?out.println(student.getSname()); Set<Course> courses = student.getCourses(); for(Course course:courses){//遍歷學生下的課程 System.?out.println(course.getCname()); } } } session.close(); } /** * 從中間表出發，班級有學生，學生修課程，故從學生角度出發進行查詢； */ @Test public?void?testManyToManyAndOneToMany_2(){ Session???session =?sessionFactory.openSession(); StringBuffer buffer =?new?StringBuffer(); buffer.append(?"from Student s left outer join fetch s.classes c? left outer join fetch s.courses cc"); Query query = session.createQuery(buffer.toString()); List<Student> studentList =?query.list(); for(Student student:studentList){ System.?out.println(student.getSname()); Classes classes = student.getClasses(); System.?out.println(classes.getCname()); Set<Course> courses = student.getCourses(); for(Course course:courses){ System.?out.println(course.getCname()); } } session.close(); } /*******************************************************************************/ /** * 面向對象的查詢 */ @Test public?void?testQueryCriteria(){ Session???session =?sessionFactory.openSession(); List<Classes>? classesList =?session.createCriteria(Classes.class).list()?; System.?out.println(classesList.size()); session.close(); } @Test public?void?testQueryCriteria_Where(){ Session???session =?sessionFactory.openSession(); Classes classes = (Classes)session.createCriteria(Classes.class).add(Restrictions.eq("cid"?, 1L)).uniqueResult(); System.?out.println(classes.getCname()); session.close(); } }

總結: 無論是一對多還是多對多，hql語句中含有fetch時，得到的list裝的是From 后面的對象，對象中可能有相關聯對象的集合或者對象；

轉載于:https://www.cnblogs.com/master-zxc/p/6737021.html

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