一、一对多
以班级Classes和学生Student为例：
回忆sql语句:
//内链接,两种方式效果一样,查询的是两边都有的数据
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;
SELECT c.cname,s.sname FROM classes c INNER JOIN student s ON s.cid=c.cid;
//左外连接，在内链接基础上，左边表有而右边表没有，两种方式等效；
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;
//右外连接，在内链接基础上，右边有而左边无，两种方式等效；
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语句：
//查询所有：
from Classes c,Student s where c.cid=s.classes.cid；
//选择某些属性查询
select c.cname,s.sname from Classes c,Student s where c.cid=s.classes.cid；
//选择某些属性，封装为bean查询；
select new cn.itheima03.hibernate.domain.ClassesView(c.cname,s.sname) from Classes c,Student s where c.cid=s.classes.cid；
//内链接查询，得到的是两个bean
from Classes c inner join c.students s；
//内敛链接查询，得到的是Classes对象，对象中包含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；
示例代码：
|
/**
* 1.一对多
* 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，注意与上句的区别；
* 得到的list是object[]，数组中的元素是Classes和Student对象；
*
*/
@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.带属性的查询；
* list中装的是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.带属性查询，将查询结果封装成一个bean；
* 得到的list中装的是classView对象；
*/
@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.内连接
* 结果与例子1一样；
*/
@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.迫切内连接1：获取所有有学生的班级及班级下的学生；
* 要想得到的集合中装的Classes对象，对象中set集合中装student，可以使用迫切内链接。
*
*/
@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.迫切内连接2
* 从学生端出发；
*/
@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.迫切内连接3：获取属性，封装结果；
* select new cn.itheima03.hibernate.domain.ClassView(c.cname,s.sname)
* from Student s inner join fetch s.classes c；
* 上述的 hql语句会报错，因为from后面想要的结构和select想要的结构是冲突的，所以
如果在from后面加fetch,不能写select语句，如果加select，不能写fetch,两者只能选择其一
*
*/
@Test
public void testOneToMany_InnerJoin_Fetch_Query_Property(){
Session session = sessionFactory.openSession();
StringBuffer buffer = new StringBuffer();
//下面的写法不对；
// 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.迫切左外连接
* 从班级出发，得到班级对应的学生
*/
@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.迫切左外连接2
* 从学生出发，得到对应的班级
*/
@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。多对多
学生Student和课程Course为例:
Student里有装Course的set集合，Course里也有装Student的set集合；
多对多与一对多操作差不多
|
/**
* 1.得到所有的学生以及其对应的课程
* 从学生端出发
* list装的是学生；
*/
@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.得到所有的课程及课程下对应的学生；
* list装的是课程
*/
@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();
}
/**
* 3.一对多和多对多的结合
* 得到所有班级下的所有学生以及所有学生下的所有课程；
* 从班级出发
*/
@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();
//去掉集合中的重复元素
Set<Classes> sets = new HashSet<Classes>(classeList);
classeList = new ArrayList<Classes>(sets);
System. out.println(classeList.size());
for(Classes classes:classeList){//遍历班级
System. out.println(classes.getCname());
Set<Student> students = classes.getStudents();//得到班级下的学生
for(Student student:students){//遍历学生
System. out.println(student.getSname());
Set<Course> courses = student.getCourses();
for(Course course:courses){//遍历学生下的课程
System. out.println(course.getCname());
}
}
}
session.close();
}
/**
* 从中间表出发，班级有学生，学生修课程，故从学生角度出发进行查询；
*/
@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();
}
/*******************************************************************************/
/**
* 面向对象的查询
*/
@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();
}
}
|
总结:
无论是一对多还是多对多，hql语句中含有fetch时，得到的list装的是From 后面的对象，对象中可能有相关联对象的集合或者对象；
来源：https://www.cnblogs.com/kingxiaozi/p/6020956.html