MyBatis implements one-to-many mapping processing

Prepare

Database table
employee table (t_employee)

department table (t_department)

table corresponding to the class
employee t_employee

Department t_department

Method 1 (collection)

Take the query of department information and the information of all employees in the department as an example.
interface

public interface DepartmentMapper {
    
    

    //通过部门的did查询部门所有信息
    Department selectDepartment(@Param("did")Integer did);

}

map file

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><!--约束，约束不同xml中所写的标签也不同-->
<mapper namespace="com.xxx.mapper.DepartmentMapper"><!--接口-->



    <resultMap id="deptMap" type="com.xxx.pojo.Department">
        <id property="did" column="did"></id>
        <result property="dname" column="dname"></result>
        <collection property="emp" ofType="com.xxx.pojo.Employee">
            <id property="eid" column="eid"></id>
            <result property="ename" column="ename"></result>
            <result property="age" column="age"></result>
            <result property="sex" column="sex"></result>
        </collection>

    </resultMap>

<!--    //通过部门的did查询部门所有信息
    Department selectDepartment(@Param("did")Integer did);-->
    <select id="selectDepartment" resultMap="deptMap">
        select t_department.*,t_employee.* from t_department left join t_employee on t_department.did=t_employee.did where t_department.did=#{did}
    </select>



</mapper>

test

		DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);

        //查询部门did=2的所有信息
        Department department = mapper.selectDepartment(2);

        System.out.println(department);

Department{did=2, dname='部门2', emp=[Employee{eid=4, ename='翠花', age=19, sex='女', dept=null}, Employee{eid=3, ename='小红', age=20, sex='女', dept=null}]}

Method 2 (step-by-step query)

Take the query of department information and the information of all employees in the department as an example.
The first interface

public interface DepartmentMapper {
    
    

    //通过部门的did查询部门所有信息
    Department selectDepartment(@Param("did")Integer did);

}

The second interface

public interface EmployeeMapper {
    
    

    //通过员工部门的did查询员工的所有信息
    Employee selectEmployee(@Param("did")Integer did);

}

First step mapping file

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><!--约束，约束不同xml中所写的标签也不同-->
<mapper namespace="com.xxx.mapper.DepartmentMapper"><!--接口-->


    <resultMap id="deptMap" type="com.xxx.pojo.Department">
        <id property="did" column="did"></id>
        <result property="dname" column="dname"></result>
        <!--此处的select中写的是第二步接口中的方法的全路径，colum中写的是第一步查出的那个数据作为第二步的参数-->
        <collection property="emp"
                    select="com.xxx.mapper.EmployeeMapper.selectEmployee"
                    column="did"></collection>
    </resultMap>

<!--    //通过部门的did查询部门所有信息
    Department selectDepartment(@Param("did")Integer did);-->
    <select id="selectDepartment" resultMap="deptMap">
        select * from t_department where did=#{did}
    </select>



</mapper>

The second step is to map the file

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><!--约束，约束不同xml中所写的标签也不同-->
<mapper namespace="com.xxx.mapper.EmployeeMapper"><!--接口-->

    <select id="selectEmployee" resultType="com.xxx.pojo.Employee">
        select * from t_employee where did=#{did}
    </select>
    
</mapper>

test

		DepartmentMapper mapper = sqlSession.getMapper(DepartmentMapper.class);

        //查询部门did=2的所有信息
        Department department = mapper.selectDepartment(2);

        System.out.println(department);

Department{did=2, dname='部门2', emp=[Employee{eid=4, ename='翠花', age=19, sex='女', dept=null}, Employee{eid=3, ename='小红', age=20, sex='女', dept=null}]}