MyBatis实现一对多映射处理

准备

数据库表
员工表（t_employee）

部门表（t_department）

表所对应的类
员工 t_employee

部门 t_department

方式一（collection）

以通过部门id，查询部门信息，以及部门中所有员工信息为例。
接口

public interface DepartmentMapper {
    
    

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

}

映射文件

<?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>

测试

		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}]}

方式二（分步查询）

以通过部门id，查询部门信息，以及部门中所有员工信息为例。
第一步接口

public interface DepartmentMapper {
    
    

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

}

第二步接口

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);

}

第一步映射文件

<?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>

第二步映射文件

<?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>

测试

		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}]}