后端框架的学习----mybatis框架（9、多对一处理和一对多处理）

9、多对一处理和一对多处理

#多对一

 <!--按照结果集嵌套查询-->
    <select id="getAllStudent1" resultMap="StudentTeacher2">
        select s.id sid,s.name sname,t.name tname
        from student s ,teacher  t
        where s.tid=t.id;
    </select>
    
    <resultMap id="StudentTeacher2" type="Student">
        <result property="id" column="sid"></result>
        <result property="name" column="sname"></result>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"></result>
        </association>
    </resultMap>

#一对多

    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid,s.name sname,t.name tname
        from student as s ,teacher as t
        where s.tid=t.id and t.id=#{tid};
    </select>

    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"></result>
        <result property="name" column="tname"></result>
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

##小结
1、关联-asscciation【多对一】
2、集合-collection 【一对多】
3、JavaType & offType

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