MyBatis implements many-to-one mapping processing

Prepare

Database table
employee table (t_employee)

department table (t_department)

table corresponding to the class
employee t_employee

Department t_department

Mode 1 (Cascade mode to deal with the mapping relationship)

Take the query of employee information and department information by employee id as an example.
interface

public interface EmployeeMapper {
    
    

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

}

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.EmployeeMapper"><!--接口-->

    <resultMap id="empMap" type="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>
        
        <result property="dept.did" column="did"></result>
        <result property="dept.dname" column="dname"></result>

    </resultMap>


    <select id="selectEmployee" resultMap="empMap">
        SELECT t_employee.*,t_department.* FROM t_employee LEFT JOIN t_department ON t_employee.did=t_department.did WHERE t_employee.eid=#{eid}
    </select>
</mapper>

test

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

        //查询员工eid=1的所有信息
        Employee employee = mapper.selectEmployee(1);

        System.out.println(employee);

Method 2 (use association to handle mapping relationship)

Take the query of employee information and department information by employee id as an example.
interface

public interface EmployeeMapper {
    
    

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

}

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.EmployeeMapper"><!--接口-->

    <resultMap id="empMap" type="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>

        <association property="dept" javaType="com.xxx.pojo.Department">
            <id property="did" column="did"></id>
            <result property="dname" column="dname"></result>
        </association>

    </resultMap>


    <select id="selectEmployee" resultMap="empMap">
        SELECT t_employee.*,t_department.* FROM t_employee LEFT JOIN t_department ON t_employee.did=t_department.did WHERE t_employee.eid=#{eid}
    </select>
</mapper>

test

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

        //查询员工eid=1的所有信息
        Employee employee = mapper.selectEmployee(1);

        System.out.println(employee);

Method 3 (step-by-step query)

Take the query of employee information and department information by employee id as an example.
interface
first step interface

public interface EmployeeMapper {
    
    

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

}

The second interface

public interface DepartmentMapper {
    
    

    //通过员工的did查询员工的所有信息
    Department selectDepartment(@Param("did")Integer did);

}

Mapping file
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.EmployeeMapper"><!--接口-->

    <resultMap id="empMap" type="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>
<!--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-->
        <association property="dept"
                     select="com.xxx.mapper.DepartmentMapper.selectDepartment"
                     column="did">
        </association>

    </resultMap>


    <select id="selectEmployee" resultMap="empMap">
        SELECT t_employee.*,t_department.* FROM t_employee LEFT JOIN t_department ON t_employee.did=t_department.did WHERE t_employee.eid=#{eid}
    </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.DepartmentMapper"><!--接口-->


    <select id="selectDepartment" resultType="com.xxx.pojo.Department">
        select * from t_department where did=#{did}
    </select>
</mapper>

test

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

        //查询员工eid=1的所有信息
        Employee employee = mapper.selectEmployee(1);

        System.out.println(employee);