HQL常用连接类型

• 内连接 inner join 或 join
• 迫切内连接 inner join fetch 或 join fetch
• 左外连接 left outer join 或 left join
• 迫切左外连接 left outer join fetch 或 left join fetch
• 右外连接 right outer join 或 right join

内连接 获取的就是两个表中的交集部分

左连接会读取左边数据表的全部数据，即使右边数据表没有对应数据。

右连接会读取右边数据表的全部数据，即使左边数据表没有对应数据。

内连接 查询部门和员工信息 from Entity [inner] join Entity.property

List<Object[]> objects=this.getCurrentSession().createQuery("from Department d join d.emps").list();
for(Object[] object:objects)
        {
            System.out.println(object[0]+"\t"+object[1]);
        }

Hibernate:
select
department0_.deptNo as deptNo0_0_,
emps1_.empNo as empNo1_1_,
department0_.deptName as deptName0_0_,
department0_.location as location0_0_,
emps1_.empName as empName1_1_,
emps1_.job as job1_1_,
emps1_.salary as salary1_1_,
emps1_.deptNo as deptNo1_1_
from
project.Department department0_
inner join
project.Emp emps1_
on department0_.deptNo=emps1_.deptNo

list集合中的每个元素都是一个Object数组,数组的第一个元素时Dept元素对象，第二个元素时Emp对象，Dept对象的emps集合元素没有被初始化，即emps集合里没有存放关联的Emp对象。Unable to evaluate the expression Method threw 'org.hibernate.LazyInitializationException' exception.

迫切内连接查询部门和员工信息from Entity [inner] join fetch Entity.property

List<Department> objects=this.getCurrentSession().createQuery("select distinct d from Department d join fetch d.emps").list();
for(Department department:objects)
        {
            System.out.println(department.getEmps());
        }

Hibernate:
select
department0_.deptNo as deptNo0_0_,
emps1_.empNo as empNo1_1_,
department0_.deptName as deptName0_0_,
department0_.location as location0_0_,
emps1_.empName as empName1_1_,
emps1_.job as job1_1_,
emps1_.salary as salary1_1_,
emps1_.deptNo as deptNo1_1_,
emps1_.deptNo as deptNo0_0__,
emps1_.empNo as empNo0__
from
project.Department department0_
inner join
project.Emp emps1_
on department0_.deptNo=emps1_.deptNo

list集合中的每个元素都是Dept对象，Hibernate使用fetch关键字实现了将Emp对象读出来后立即填充到对应的Dept对象的集合属性中。如果有一些Dept对象，可以使用select distinct d from Dept d inner join fetch d.emps解决。

左外连接 from Entity left [outer] join Entity.property

List<Object[]> objects=this.getCurrentSession().createQuery("from Department d left join d.emps").list();
 for (Object[] object:objects)
        {
            System.out.println(object[0]+"\t"+object[1]);
        }

Hibernate:
select
department0_.deptNo as deptNo0_0_,
emps1_.empNo as empNo1_1_,
department0_.deptName as deptName0_0_,
department0_.location as location0_0_,
emps1_.empName as empName1_1_,
emps1_.job as job1_1_,
emps1_.salary as salary1_1_,
emps1_.deptNo as deptNo1_1_
from
project.Department department0_
left outer join
project.Emp emps1_
on department0_.deptNo=emps1_.deptNo

迫切左外连接from Entity left [outer] join fetch Entity.property

List<Department> objects=this.getCurrentSession().createQuery("select distinct d from Department d left join fetch d.emps").list();

for (Department department:objects)
        {
            System.out.println(department.getEmps());
        }

Hibernate:
select
distinct department0_.deptNo as deptNo0_0_,
emps1_.empNo as empNo1_1_,
department0_.deptName as deptName0_0_,
department0_.location as location0_0_,
emps1_.empName as empName1_1_,
emps1_.job as job1_1_,
emps1_.salary as salary1_1_,

    emps1_.deptNo as deptNo1_1_,
    emps1_.deptNo as deptNo0_0__,
    emps1_.empNo as empNo0__ 
from
    project.Department department0_ 
left outer join
    project.Emp emps1_ 
        on department0_.deptNo=emps1_.deptNo

右外连接 from Entity right [outer] join Entity.property

List<Object[]> objects=this.getCurrentSession().createQuery("from Department d right join d.emps").list();
 for (Object[] object:objects)
        {
            System.out.println(object[0]+"\t"+object[1]);
        }

Hibernate:
select
department0_.deptNo as deptNo0_0_,
emps1_.empNo as empNo1_1_,
department0_.deptName as deptName0_0_,
department0_.location as location0_0_,
emps1_.empName as empName1_1_,
emps1_.job as job1_1_,
emps1_.salary as salary1_1_,
emps1_.deptNo as deptNo1_1_
from
project.Department department0_
right outer join
project.Emp emps1_
on department0_.deptNo=emps1_.deptNo

隐式内连接 按部门条件查询员工信息

 List<Emp> emps=this.getCurrentSession().createQuery("from Emp e where e.department.deptNo=:deptNo")
                .setParameter("deptNo",deptNo)
                .list();

Hibernate:
select
emp0_.empNo as empNo1_,
emp0_.empName as empName1_,
emp0_.job as job1_,
emp0_.salary as salary1_,
emp0_.deptNo as deptNo1_
from
project.Emp emp0_
where
emp0_.deptNo=?
Hibernate:
select
department0_.deptNo as deptNo0_0_,
department0_.deptName as deptName0_0_,
department0_.location as location0_0_
from
project.Department department0_
where
department0_.deptNo=?

List<Object[]> objects=this.getCurrentSession().createQuery(" select e.empName,e.department.deptName from Emp  e").list();

Hibernate:
select
emp0_.empName as col_0_0_,
department1_.deptName as col_1_0_
from
project.Emp emp0_,
project.Department department1_
where
emp0_.deptNo=department1_.deptNo