mybatis处理一对多的查询

//查询出某个班级对应的所有老师和学生

1、使用嵌套结果

<select id="findClasses3" parameterType="int" resultMap="findClasses3Map">
    select c.*,t.*,s.* from classes c,teacher t,student s where c.t_id = t.t_id and s.c_id = c.c_id and c.c_id = #{id}
</select>
<resultMap type="Classes" id="findClasses3Map">
    <id property="id" column="c_id"/>
    <result property="name" column="c_name"/>
    <association property="teacher" javaType="Teacher">
        <id property="id" column="t_id"/>
        <result property="name" column="t_name"/>
    </association>
    <collection property="students" ofType="Student">
        <id property="id" column="s_id"/>
        <result property="name" column="s_name"/>
    </collection>
</resultMap>

使用collection节点对于list类型的返回使用ofType

2、使用嵌套查询

<select id="findClasses4" parameterType="int" resultMap="findClasses4Map">
    select * from classes where c_id=#{id}
</select>
<resultMap type="Classes" id="findClasses4Map">
    <id property="id" column="c_id"/>
    <result property="name" column="c_name"/>
    <association property="teacher" select="findTeacher" column="t_id">
    </association>
    <collection property="students" select="findStudents" column="c_id"></collection>
</resultMap>
<select id="findStudents" parameterType="int" resultType="Student">
    select s_id id,s_name name from student where c_id = #{id}
</select>

<select id="findTeacher" parameterType="int" resultType="Teacher">
    select t_id id,t_name name from teacher where t_id = #{t_id}
</select>

使用collection节点对于list返回直接用一个普通查询可以实现！