多对一查询

方法一:

<select id="getStudent" resultMap="StudentTeacher">
        select * from student
    </select>
    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--association关联属性 property属性名 javaType属性类型 column在多的一方的表中的列名-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getTeacher" resultType="teacher">
        select * from teacher where id = #{id}
    </select>

方法二：

<select id="getStudent2" resultMap="StudentTeacher2">
        select s.`id` sid,s.`name` sname,t.`name` tname,t.`id` tid,s.tid stid
        from student s,teacher t
        where s.tid=t.id
    </select>
    <resultMap id="StudentTeacher2" type="Student">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <result property="tid" column="stid"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
            <result property="id" column="tid"/>
        </association>
    </resultMap>

一对多

<mapper namespace="com.skl.dao.TeacherMapper">
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.name tname,t.id tis
        from teacher t,student s
        where s.tid = t.id and t.id = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>
</mapper>

注意

1. 关联 - assocation（多对一）
2. 集合 - collection （一对多）

3. javaType & ofType

   1. javaType用来指定实体类中属性的类型
   2. ofType用来指定映射带List或者集合中的pojo类型，泛型中的约束类型