Mybatis联合查询的实现方法

2022-01-07 11:35:40 作者：WorkHaH

本文主要介绍了 Mybatis联合查询的实现方法，文中通过示例代码介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们可以参考一下

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