Mybatis学习笔记-一对一关联

public class Teacher {
private int id;
private String name;
//...
}

public class Classes {
private int id;
private String name;
private Teacher teacher;
//...
}

<!--
方式一：嵌套结果：使用嵌套结果映射来处理重复的联合结果的子集
封装联表查询的数据(去除重复的数据)
select * from class c, teacher t where c.teacher_id=t.t_id and  c.c_id=1
-->
<select id="getClass" parameterType="int" resultMap="ClassResultMap">
select * from class c, teacher t where c.teacher_id=t.t_id and  c.c_id=#{id}
</select>
<resultMap type="com.mybatis.test03.Classes" id="ClassResultMap">
<id property="id" column="c_id"/>
<result property="name" column="c_name"/>
<association property="teacher" javaType="com.mybatis.test03.Teacher">
<id property="id" column="t_id"/>
<result property="name" column="t_name"/>
</association>
</resultMap>

<!-- 方式二：嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型 SELECT * FROM class WHERE c_id=1;
SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值 -->
<select id="getClass2" parameterType="int" resultMap="ClassResultMap2">
select * from class where c_id=#{id}
</select>
<resultMap type="com.mybatis.test03.Classes" id="ClassResultMap2">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" column="teacher_id"
select="getTeacher"><!-- 映射到后面的getTeacher -->
</association>
</resultMap>
<select id="getTeacher" parameterType="int" resultType="com.mybatis.test03.Teacher">
SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id}
</select>