Hibernate多表查询连接操作
SQL多表操作分类;
1.交叉连接:select*from t_customer cross Join t_order;
2.显示内连接: select*from t_customer c inner join t_order o on c.id=o.c_customer_id;
隐示内连接:select*from t_customer c ,t_order o WHERE c.id=o.c_customer_id; where可消除笛卡尔积
笛卡尔积是两个表任意组合,查询出两表全部的组合,但我们只想查询某一种组合,加where限制条件可消除笛卡尔积.
3.外链接:select*from t_customer c left join t_order o ON c.id=o.c_customer_id;左外:把左边的全部显示出来.
Hql多表操作分类:
1.交叉连接 ::select*from t_customer cross Join t_order;
2.内连接
a) 显示内连接: b) 隐式内连接 c) 迫切内连接
//显示内连接---结果Customer的id=1的属性,order属性;Customer的id=1的属性,order属性
@Test
public void test010(){
Session session = HibernateUtils.openSession();
session.beginTransaction();
String hql="from Customer c inner join c.orders with c.id=1";
Query query = session.createQuery(hql);
List<Object[]> list = query.list();
for (Object[] objects : list) {
for (Object object : objects) {
System.out.println(object );
}
}
session.getTransaction();
session.close();
}
//隐示内连接----消除了笛卡尔迪.它是通过"."运算符来关联.
@Test
public void test011(){
Session session = HibernateUtils.openSession();
session.beginTransaction();
String hql="from Order o where o.c.id=1";
Query query = session.createQuery(hql);
List list = query.list();
System.out.println(list);
session.getTransaction();
session.close();
}
//迫切内连接(立即加载)----distinct c 消除了笛卡尔迪,迫切内连接得到的结果是直接封装到po类,而内连接得到的结果是Object[],数组中封装的po类.
@Test
public void test012(){
Session session = HibernateUtils.openSession();
session.beginTransaction();
//底层也是inner join,只不过把结果封装到对象中.
String hql=" select distinct c from Customer c inner join fetch c.orders ";
Query query = session.createQuery(hql);
List<Customer> list = query.list();//集合中装入的是from后面的对象
for (Customer c : list) {
System.out.println(c);
}
session.getTransaction();
session.close();
}
3.外连接
1)左外连接 2)迫切左外连接 3)右外连接
//外链接
@Test
public void test013(){
Session session = HibernateUtils.openSession();
session.beginTransaction();
//底层也是inner join,只不过把结果封装到对象中.
String hql=" from Customer c left join c.orders ";
Query query = session.createQuery(hql);
List<Object[]>list = query.list();
for (Object[] objects : list) {
for (Object object : objects) {
System.out.println(object+"\t");
}
System.out.println();
}
session.getTransaction();
session.close();
}
//迫切左外链接
@Test
public void test014(){
Session session = HibernateUtils.openSession();
session.beginTransaction();
//底层也是inner join,只不过把结果封装到对象中.
String hql=" select distinct c from Customer c left join fetch c.orders where c.id=1";
Query query = session.createQuery(hql);
List<Customer> list = query.list();
/*for (Customer c : list) {
System.out.println(c);
}*/
System.out.println(list);
session.getTransaction();
session.close();
}
注意:在Hibernate中有迫切连接,在sql中没有.