mybatis 一对一查询的几种方式

<mapper namespace="com.itheima.mapper.TbStudentMapper">

    <!--一对一关联查询，使用级联属性，其中tbTeacher是TbStudent的属性-->
    <resultMap id="myStuAndTea" type="com.itheima.pojo.TbStudent">
        <id column="u_id" property="uId"></id>
        <result column="u_name" property="uName"></result>
        <result column="sex" property="sex" ></result>
        <result column="t_id" property="tId"></result>
        <result column="id" property="tbTeacher.id"></result>
        <result column="t_name" property="tbTeacher.tName"></result>
    </resultMap>

<select id="getStuAndTea" resultMap="myStuAndTea">
    select * from tb_student stu,tb_teacher tch where stu.t_id=tch.id and stu.u_id=#{id}
</select>
</mapper>

DEBUG [main] - ==>  Preparing: select * from tb_student stu,tb_teacher tch where stu.t_id=tch.id and stu.u_id=? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - <==      Total: 1
TbStudent{uId=1, uName='zhangsan', sex='男', tId=101, tbTeacher=TbTeacher{id=101, tName='admin'}}

 方式二：

<mapper namespace="com.itheima.mapper.TbStudentMapper">

    <!--一对一关联查询，使用association对于单个对象的关联查询，tbTeacher是TbStudent的属性，javaType不可缺少-->
    <resultMap id="myStuAndTea" type="com.itheima.pojo.TbStudent">
        <id column="u_id" property="uId"></id>
        <result column="u_name" property="uName"></result>
        <result column="sex" property="sex" ></result>
        <result column="t_id" property="tId"></result>
        <association property="tbTeacher" javaType="com.itheima.pojo.TbTeacher">
            <id column="id" property="id"></id>
            <result column="t_name" property="tName"></result>
        </association>
    </resultMap>

<select id="getStuAndTea" resultMap="myStuAndTea">
    select * from tb_student stu,tb_teacher tch where stu.t_id=tch.id and stu.u_id=#{id}
</select>

</mapper>

 方式三：

<mapper namespace="com.itheima.mapper.TbStudentMapper">

    <!--一对一关联查询，使用association分步查询，前提，在TbTeacherMapper.xml的sql映射中有getTeacher的查询方法-->
　　　<!--column的值是TbStudent中传过来的值，执行了两次查询-->

    <resultMap id="myStuAndTea" type="com.itheima.pojo.TbStudent">
        <id column="u_id" property="uId"></id>
        <result column="u_name" property="uName"></result>
        <result column="sex" property="sex" ></result>
        <result column="t_id" property="tId"></result>
        <association property="tbTeacher" select="com.itheima.mapper.TbTeacherMapper.getTeacher" column="t_id">

        </association>
    </resultMap>

<select id="getStuAndTea" resultMap="myStuAndTea">
    select * from tb_student stu where  stu.u_id=#{id}
</select>

</mapper>
DEBUG [main] - ==>  Preparing: select * from tb_student stu where stu.u_id=? 
DEBUG [main] - ==> Parameters: 1(Integer)
DEBUG [main] - ====>  Preparing: select id,t_name as tName from tb_teacher where id=? 
DEBUG [main] - ====> Parameters: 101(Integer)
DEBUG [main] - <====      Total: 1
DEBUG [main] - <==      Total: 1
TbStudent{uId=1, uName='zhangsan', sex='男', tId=101, tbTeacher=TbTeacher{id=101, tName='admin'}}DEBUG