Hibernate（十四）：HQL查询（三）

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。