背景

基于上两章节《Hibernate(十二):HQL查询(一)》、《Hibernate(十三):HQL查询(二)》,已经学习了一部分关于HQL的用法:

  1. HQL带参数查询
  2. HQL Order By排序查询
  3. HQL 设置实体参数查询
  4. HQL分页查询
  5. HQL命名语句查询
  6. HQL投影查询
  7. HQL报表查询

本章节将会学习:HQL(迫切)左外连接、HQL(迫切)内连接、总结关联级别运行时的检索策略。

HQL(迫切)左外连接

 迫切左外连接

1)LEFT JOIN FETCH 关键字表示迫切左外连接检索策略

2)list()方法返回的集合中存放实体对象的引用,每个Department对象关联的Employee结合都被初始化,存放所有关联的Employee的实体对象

3)查询结果中可能会被包含重复的元素,可以通过HashSet来过滤重复元素,也可以在hql语句中使用distinct.

测试代码:

     @Test
public void testLeftJoinFetch() {
String hql = "FROM Department d LEFT JOIN FETCH d.employees";
List<Department> departs = session.createQuery(hql).list(); System.out.println("直接左外连接查询结果集合个数:" + departs.size());
Department depart = departs.get(0);
System.out.println("直接左外连接查询结果集的第一个元素Department Name:" + depart.getName() + ",该department的Employees个数:" + depart.getEmployees().size()); departs = new ArrayList<>(new LinkedHashSet<>(departs));
System.out.println("使用LinkedHashSet排除重复项后集合个数:" + departs.size()); hql = "Select Distinct d FROM Department d LEFT JOIN FETCH d.employees";
List<Department> departs2 = session.createQuery(hql).list();
System.out.println("使用Distinct排除重复项后集合个数:" + departs2.size());
for(Department department : departs2){
System.out.println("使用Distinct排除重复项后元素Department Name:" + department.getName() + ",该department的Employees个数:" + department.getEmployees().size());
}
}

执行sql及结果:

 Hibernate:
select
department0_.ID as ID1_0_0_,
employees1_.ID as ID1_1_1_,
department0_.NAME as NAME2_0_0_,
employees1_.NAME as NAME2_1_1_,
employees1_.SALARY as SALARY3_1_1_,
employees1_.EMAIL as EMAIL4_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_0__,
employees1_.ID as ID1_1_0__
from
DX_DEPARTMENT department0_
left outer join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
直接左外连接查询结果集合个数:80
直接左外连接查询结果集的第一个元素Department Name:开发部门,该department的Employees个数:16
使用LinkedHashSet排除重复项后集合个数:5
Hibernate:
select
distinct department0_.ID as ID1_0_0_,
employees1_.ID as ID1_1_1_,
department0_.NAME as NAME2_0_0_,
employees1_.NAME as NAME2_1_1_,
employees1_.SALARY as SALARY3_1_1_,
employees1_.EMAIL as EMAIL4_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_0__,
employees1_.ID as ID1_1_0__
from
DX_DEPARTMENT department0_
left outer join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
使用Distinct排除重复项后集合个数:5
使用Distinct排除重复项后元素Department Name:开发部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:测试部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:业务部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:财务部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:行政部门,该department的Employees个数:16

左外连接

1)LEFT JOIN 关键字表示左外连接查询

2)list()方法返回的集合中存放的是数组类型

3)根据配置文件来决定Employee集合的策略(以下测试代码,前提是employee.hbm.xml中没有配置join='fetch'策略)

4)如果希望list()方法返回的集合中仅包含Department对象,可以在HQL查询语句中使用SELECT关键字。

测试代码:

     @Test
public void testLeftJoin() {
String hql = "FROM Department d LEFT JOIN d.employees";
List<Department> departs = session.createQuery(hql).list(); System.out.println("直接左外连接查询结果集合个数:" + departs.size());
// 将会抛出异常:
//Department depart = departs.get(0);
//System.out.println("直接左外连接查询结果集的第一个元素Department Name:" + depart.getName() + ",该department的Employees个数:" + depart.getEmployees().size()); // 排重无效
departs = new ArrayList<>(new LinkedHashSet<>(departs));
System.out.println("使用LinkedHashSet排除重复项后集合个数:" + departs.size()); hql = "Select Distinct d FROM Department d LEFT JOIN d.employees";
List<Department> departs2 = session.createQuery(hql).list();
System.out.println("使用Distinct排除重复项后集合个数:" + departs2.size());
for(Department department : departs2){
System.out.println("使用Distinct排除重复项后元素Department Name:" + department.getName() + ",该department的Employees个数:" + department.getEmployees().size());
}
}

执行sql及结果

 Hibernate:
select
department0_.ID as ID1_0_0_,
employees1_.ID as ID1_1_1_,
department0_.NAME as NAME2_0_0_,
employees1_.NAME as NAME2_1_1_,
employees1_.SALARY as SALARY3_1_1_,
employees1_.EMAIL as EMAIL4_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_DEPARTMENT department0_
left outer join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
直接左外连接查询结果集合个数:80
使用LinkedHashSet排除重复项后集合个数:80
Hibernate:
select
distinct department0_.ID as ID1_0_,
department0_.NAME as NAME2_0_
from
DX_DEPARTMENT department0_
left outer join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
使用Distinct排除重复项后集合个数:5
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:开发部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:测试部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:业务部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:财务部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:行政部门,该department的Employees个数:16

HQL(迫切)内连接

 迫切内连接

1)INNER JOIN FETCH关键字表示迫切内连接,也可以省略INNER关键字

2)list()方法返回的集合中存放Department对象的引用,每个Department对象的Employee集合都被初始化,存放所有关联的Employee对象。

测试函数:

     @Test
public void testInnerJoinFetch() {
// // 先执行追加employee一条记录。
// Employee employee = new Employee("name", 10000, "email@test.com");
// session.save(employee);
// Department department = new Department("党政部门");
// session.save(department); String hql = "FROM Department d INNER JOIN FETCH d.employees";
List<Department> departs = session.createQuery(hql).list(); System.out.println("直接左外连接查询结果集合个数:" + departs.size());
Department depart = departs.get(0);
System.out.println("直接左外连接查询结果集的第一个元素Department Name:" + depart.getName() + ",该department的Employees个数:" + depart.getEmployees().size()); departs = new ArrayList<>(new LinkedHashSet<>(departs));
System.out.println("使用LinkedHashSet排除重复项后集合个数:" + departs.size()); hql = "Select Distinct d FROM Department d INNER JOIN FETCH d.employees";
List<Department> departs2 = session.createQuery(hql).list();
System.out.println("使用Distinct排除重复项后集合个数:" + departs2.size());
for (Department department : departs2) {
System.out.println("使用Distinct排除重复项后元素Department Name:" + department.getName() + ",该department的Employees个数:" + department.getEmployees().size());
}
}

执行sql及结果:

 Hibernate:
select
department0_.ID as ID1_0_0_,
employees1_.ID as ID1_1_1_,
department0_.NAME as NAME2_0_0_,
employees1_.NAME as NAME2_1_1_,
employees1_.SALARY as SALARY3_1_1_,
employees1_.EMAIL as EMAIL4_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_0__,
employees1_.ID as ID1_1_0__
from
DX_DEPARTMENT department0_
inner join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
直接左外连接查询结果集合个数:80
直接左外连接查询结果集的第一个元素Department Name:开发部门,该department的Employees个数:16
使用LinkedHashSet排除重复项后集合个数:5
Hibernate:
select
distinct department0_.ID as ID1_0_0_,
employees1_.ID as ID1_1_1_,
department0_.NAME as NAME2_0_0_,
employees1_.NAME as NAME2_1_1_,
employees1_.SALARY as SALARY3_1_1_,
employees1_.EMAIL as EMAIL4_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_0__,
employees1_.ID as ID1_1_0__
from
DX_DEPARTMENT department0_
inner join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
使用Distinct排除重复项后集合个数:5
使用Distinct排除重复项后元素Department Name:开发部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:测试部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:业务部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:财务部门,该department的Employees个数:16
使用Distinct排除重复项后元素Department Name:行政部门,该department的Employees个数:16

内连接

1)INNER JOIN 关键字表示内连接,也可以省略INNER 关键字

2)list()方法的集合中存放的每个元素对应查询结果的一条记录,每个元素都是对象数组

3)如果希望list()方法的返回集合仅包含Department对象,可以在HQL查询语句中使用SELECT关键字。

测试函数:

     @Test
public void testInnerJoin() {
String hql = "FROM Department d INNER JOIN d.employees";
List<Department> departs = session.createQuery(hql).list(); System.out.println("直接左外连接查询结果集合个数:" + departs.size());
// 将会抛出异常:
// Department depart = departs.get(0);
// System.out.println("直接左外连接查询结果集的第一个元素Department Name:"
// +depart.getName() + ",该department的Employees个数:" +
// depart.getEmployees().size()); // 排重无效
departs = new ArrayList<>(new LinkedHashSet<>(departs));
System.out.println("使用LinkedHashSet排除重复项后集合个数:" + departs.size()); hql = "Select Distinct d FROM Department d INNER JOIN d.employees";
List<Department> departs2 = session.createQuery(hql).list();
System.out.println("使用Distinct排除重复项后集合个数:" + departs2.size());
for (Department department : departs2) {
System.out.println("使用Distinct排除重复项后元素Department Name:" + department.getName() + ",该department的Employees个数:" + department.getEmployees().size());
}
}

执行sql及结果:

 Hibernate:
select
department0_.ID as ID1_0_0_,
employees1_.ID as ID1_1_1_,
department0_.NAME as NAME2_0_0_,
employees1_.NAME as NAME2_1_1_,
employees1_.SALARY as SALARY3_1_1_,
employees1_.EMAIL as EMAIL4_1_1_,
employees1_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_DEPARTMENT department0_
inner join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
直接左外连接查询结果集合个数:80
使用LinkedHashSet排除重复项后集合个数:80
Hibernate:
select
distinct department0_.ID as ID1_0_,
department0_.NAME as NAME2_0_
from
DX_DEPARTMENT department0_
inner join
DX_EMPLOYEE employees1_
on department0_.ID=employees1_.DEPARTMENT_ID
使用Distinct排除重复项后集合个数:5
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:开发部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:测试部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:业务部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:财务部门,该department的Employees个数:16
Hibernate:
select
employees0_.DEPARTMENT_ID as DEPARTME5_1_0_,
employees0_.ID as ID1_1_0_,
employees0_.ID as ID1_1_1_,
employees0_.NAME as NAME2_1_1_,
employees0_.SALARY as SALARY3_1_1_,
employees0_.EMAIL as EMAIL4_1_1_,
employees0_.DEPARTMENT_ID as DEPARTME5_1_1_
from
DX_EMPLOYEE employees0_
where
employees0_.DEPARTMENT_ID=?
使用Distinct排除重复项后元素Department Name:行政部门,该department的Employees个数:16

总结关联级别运行时的检索策略

1)如果在HQL中没有显式指定检索策略,将使用映射文件配置的检索策略

2)HQL会忽略映射文件中设置的迫切左外连接检索策略,如果HQL采用迫切左外连接策略,就必须在HQL查询语句中显式的指定它

3)若在HQL代码中显式指定了检索策略,就会覆盖映射文件中配置的检索策略。

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