mybatis的association用法，表示has-one

前提：Student(id,name,supervisor_id), Teacher(id,name)，其中student.supervisor_id关联teacher.id，表示一个学生只有一个导师，一对一关系。

重要说明：column属性对应的SQL语句中，定义的别名（如果定义的话）

写法一：嵌套的select语句
<resultMap>
<association property="supervisor" column="supervisor_id" 
select="findTeacherById" />
</resultMap>

<select id="findTeacherById" parameterType="int" resultMap="TeacherResultMap" >
    select *
    from   t_Teacher
    where   id=#{id}
</select>

<select id="findStudentById" parameterType="int" resultMap="StudentResultMap" >
    select  *
    from    t_Student
    where    id=#{id}
</select>

写法二： 嵌套的resultMap，直接在association中映射t_teacher表
<resultMap>
<association property="supervisor" javaType="Teacher">
    <id property="id" column="t_id"/>
    <result property="name" column="t_name"/>
</association>
</resultMap>

<select id="findSuperVisorByStudent" parameterType="int" resultMap="StudentResultMap">
    select  ts.id as s_id,
   ts.name as s_name,
   ts.supervisor_id as s_supervisor_id,
   tt.id as t_id,
   tt.name as t_name
    from    t_student ts left outer join t_teacher tt
    on    ts.supervisor_id = tt.id
    where   ts.id = #{id}
</select>

写法三： 嵌套的resultMap，在外部映射t_teacher表
<resultMap>
<association property="supervisor" resultMap="supervisorResultMap" />
</resultMap>

<resultMap type="Teacher" id="supervisorResultMap">
    <id property="id" column="t_id"/>
    <result property="name" column="t_name"/>
</resultMap>

<select id="findSuperVisorByStudent" parameterType="int" resultMap="StudentResultMap">
    select  ts.id as s_id,
   ts.name as s_name,
   ts.supervisor_id as s_supervisor_id,
   tt.id as t_id,
   tt.name as t_name
    from    t_student ts left outer join t_teacher tt
    on    ts.supervisor_id = tt.id
    where   ts.id = #{id}
</select>