Mybatis联合查询的实现方法

数据库表结构

department

employee

要求一

现在的要求是输入 id 把 employee 表的对应员工数据查询出来，并且查询出该员工的所处部门信息

public class Employee {
   private Integer id;
   private String lastName;
   private String email;
   private String gender;
   private Department dept;
setter和getter.......
}

public class Department {
   private Integer id;
   private String departmentName;
   setter和getter.......
}

1、级联属性封装结果集

实现

这个要求很明显就要用到两个表，想要把部门信息封装到Employee对象的dept字段需要用到resultMap属性

方法一

<!-- public Employee getEmployee(int id); -->
<select id="getEmployee" resultMap="emp1">
select e.*, d.id did, d.department_name
from employee e,
department d
where e.d_id = d.id
and e.id = #{id}
</select>
<resultMap id="emp1" type="employee">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<result column="did" property="dept.id"/>
<result column="department_name" property="dept.departmentName"/>
</resultMap>

方法二

<!-- public Employee getEmployee(int id); -->
<select id="getEmployee" resultMap="emp2">
select e.*, d.id did, d.department_name
from employee e,
department d
where e.d_id = d.id
and e.id = #{id}
</select>
<resultMap id="emp2" type="employee">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<association property="dept" javaType="department">
<id column="did" property="id"/>
<result column="department_name" property="departmentName"/>
</association>
</resultMap>

测试

@Test
   public void test1() {
       SqlSession sqlSession = MyTest.getSqlSession();
       EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
       System.out.println(mapper.getEmployee(1));
   }

结果

2、分步查询

方法

DepartmentMapper.xml

<!-- public Department getDepartment2(int id); -->
<select id="getDepartment2" resultType="department">
select * from department where id = #{id}
</select>

EmployeeMaper.xml

<!-- public Employee getEmployee2(int id); -->
<!-- 分步查询 -->
<select id="getEmployee2" resultMap="emp3">
select * from employee where id = #{id}
</select>
<resultMap id="emp3" type="employee">
<id column="id" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
<association property="dept" select="com.workhah.mapper.department.DepartmentMapper.getDepartment2" column="d_id"/>
</resultMap>

测试

@Test
   public void test1() {
       SqlSession sqlSession = MyTest.getSqlSession();
       EmployeeMapper mapper = sqlSession.getMapper(EmployeeMapper.class);
       System.out.println(mapper.getEmployee2(1));
   }

结果

要求二

现在的要求是输入 id 把 department 表对应的部门信息查询出来，并且查询该部门下的所有员工信息

public class Employee {
   private Integer id;
   private String lastName;
   private String email;
   private String gender;
setter和getter.......
}

public class Department {
   private Integer id;
   private String departmentName;
   private List<Employee> employees;
   setter和getter.......
}

3、级联属性封装结果集

方法

<!--   public Department getDepartment(int id); -->
<select id="getDepartment" resultMap="dep1">
select d.*, e.id eid, e.last_name, e.email, e.gender
from department d
left join employee e on d.id = e.d_id
where d.id = #{id}
</select>
<resultMap id="dep1" type="department">
<id column="id" property="id"/>
<result column="department_name" property="departmentName"/>
<collection property="employees" ofType="employee">
<id column="eid" property="id"/>
<result column="last_name" property="lastName"/>
<result column="email" property="email"/>
<result column="gender" property="gender"/>
</collection>
</resultMap>

测试

@Test
   public void test2() {
       SqlSession sqlSession = MyTest.getSqlSession();
       DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);
       System.out.println(mapper.getDepartment(1));
   }

结果

4、分步查询

EmployeeMaper.xml

<!--  public List<Employee> getEmployeeByDid(int did); -->
<select id="getEmployeeByDid" resultType="employee">
select *
from employee
where d_id = #{did}
</select>

DepartmentMapper.xml

<!-- public Department getDepartment3(int id); -->
<select id="getDepartment3" resultMap="dep2">
select *
from department
where id = #{id}
</select>
<resultMap id="dep2" type="department">
<id column="id" property="id"/>
<result column="depart_name" property="departName"/>
<collection property="employees" ofType="employee"
select="com.workhah.mapper.employee.EmployeeMapper.getEmployeeByDid" column="id"/>
</resultMap>

测试

@Test
   public void test2() {
       SqlSession sqlSession = MyTest.getSqlSession();
       DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);
       System.out.println(mapper.getDepartment3(1));
   }

结果

来源：https://www.cnblogs.com/workhah/p/15759172.html