1.准备数据

2.多对一

主表和副表
多对一：多的一方为主表，一的一方为副表。
一对多：一的一方为主表，多的一方为副表。

2.1 方法一：级联属性映射

<mapper namespace="com.sdnu.mybatis.mapper.StudentMapper">
    <resultMap id="studentResultMap" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <result property="clazz.cid" column="cid"/>
        <result property="clazz.cname" column="cname"/>
    </resultMap>
    <select id="selectById" resultMap="studentResultMap">
        select
            s.sid, s.sname, c.cid, c.cname
        from
            t_stu s left join t_clazz c on s.cid = c.cid
        where
            s.sid = #{id}
    </select>
</mapper>

2.2 方法二：association

    <resultMap id="studentResultMapAssociation" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <!--
            property:提供要映射的POJO类属性名
            javaType:用来指定要映射的java类型
        -->
        <association property="clazz" javaType="Clazz">
            <id property="cid" column="cid"/>
            <result property="cname" column="cname"/>
        </association>
    </resultMap>
    <select id="selectByIdAssociation" resultMap="studentResultMapAssociation">
        select
            s.sid, s.sname, c.cid, c.cname
        from
            t_stu s left join t_clazz c on s.cid = c.cid
        where
            s.sid = #{id}
    </select>

2.3 方法三：分步查询

StudentMapper.java

public interface StudentMapper {
    Student selectByIdStep1(Integer sid);
}

StudentMapper.xml

<mapper namespace="com.sdnu.mybatis.mapper.StudentMapper">
    <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <association property="clazz"
                     select="com.sdnu.mybatis.mapper.ClazzMapper.selectByIdStep2"
                     column="cid"/>
    </resultMap>
    <select id="selectByIdStep1" resultMap="studentResultMapByStep">
        select sid, sname, cid from t_stu where sid = #{sid}
    </select>
</mapper>

ClazzMapper.java

public interface ClazzMapper {
    Clazz selectByIdStep2(Integer cid);
}

ClazzMapper.xml

<mapper namespace="com.sdnu.mybatis.mapper.ClazzMapper">
    <select id="selectByIdStep2" resultType="Clazz">
        select cid, cname from t_clazz where cid = #{cid}
    </select>
</mapper>

分步查询好处：
● 第一个优点：代码复用性增强。
● 第二个优点：支持延迟加载。【暂时访问不到的数据可以先不查询。提高程序的执行效率。】

2.4 一对多延迟加载

fetchType=“lazy”

    <resultMap id="studentResultMapByStep" type="Student">
        <id property="sid" column="sid"/>
        <result property="sname" column="sname"/>
        <association property="clazz"
                     select="com.sdnu.mybatis.mapper.ClazzMapper.selectByIdStep2"
                     column="cid"
                     fetchType="lazy"/>
    </resultMap>
    <select id="selectByIdStep1" resultMap="studentResultMapByStep">
        select sid, sname, cid from t_stu where sid = #{sid}
    </select>

一般全局开启延迟加载

    <settings>
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

不需要时再关掉：
fetchType=“eager”

3. 一对多

3.1 方法一：collection

    <resultMap id="clazzResultMap" type="Clazz">
        <id property="cid" column="cid"/>
        <result property="cname" column="cname"/>
        <!--ofType属性用来指定集合中的元素类型-->
        <collection property="stus" ofType="Student">
            <id property="sid" column="sid"/>
            <result property="sname" column="sname"/>
        </collection>
    </resultMap>
    <select id="selectByCollection" resultMap="clazzResultMap">
        select s.sid, s.sname, c.cid, c.cname from t_clazz c left join t_stu s on c.cid = s.cid where c.cid = #{cid}
    </select>

3.2 方法二：分步查询

ClazzMapper.java

public interface ClazzMapper {
    Clazz selectByStep1(Integer cid);
}

ClazzMapper.xml

    <resultMap id="clazzResultMapStep" type="Clazz">
        <id property="cid" column="cid"/>
        <result property="cname" column="cname"/>
        <collection property="stus"
                    select="com.sdnu.mybatis.mapper.StudentMapper.selectByCidStep2"
                    column="cid"/>
    </resultMap>
    <select id="selectByStep1" resultMap="clazzResultMapStep">
        select cid, cname from t_clazz where cid = #{cid}
    </select>

StudentMapper.java

public interface StudentMapper {
	List<Student> selectByCidStep2(Integer cid);
}

StudentMapper.xml

<mapper namespace="com.sdnu.mybatis.mapper.StudentMapper">
    <select id="selectByCidStep2" resultType="Student">
        select * from t_stu where cid = #{cid}
    </select>
</mapper>