以案例说明(以下案例代码都敲过验证过)

 

多对一（多个学生对一个老师  即学生集合中都存一个老师对象）

Mybatis多对一实现方式1：

//定义Student 和 Teacher 实体
@Data
public class Student {
    private int id;
    private String name;
    //tid对应teacher的id
    private int tid;
    private Teacher teacher;
}

@Data
public class Teacher {
    private int id;
    private String name;
}

<mapper namespace="mapper.StudentMapper">
    <resultMap id="stuAboutTea" type="bean.Student">
        <result property="id" column="ic"></result>
        <result property="name" column="name"></result>
        //普通查询只需查询selectstudent就好了
        //多对一查询多了association标签  我认为这个标签作用就是嵌套查询
        //其中javaType是property中teacher对象的类 select嵌套查询的id
        <association property="teacher" column="tid" javaType="bean.Teacher" select="selectstubyid"/>
    </resultMap>
    
    <select id="selectstudent" resultMap="stuAboutTea">
        select * from student
    </select>

    <select id="selectstubyid" resultType="bean.Teacher">
        select * from teacher where id=#{id}
    </select>
</mapper>              

这种方式调selectid为selectstudent的sql  selectstudent中就嵌套了selectstubyid的查询  嵌套的结果集存在private Teacher teacher;

 

Mybatis多对一实现方式2：

   <resultMap id="StudentTeacher2" type="bean.Student">
        <id property="id" column="sid"/>
        <result property="name" column="sname"/>
        <!--关联对象property 关联对象在Student实体类中的属性-->
        <association property="teacher" javaType="bean.Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

    <select id="getStudents2" resultMap="StudentTeacher2" >
        select s.id sid, s.name sname , t.name tname
        from student s,teacher t
        where s.tid = t.id
    </select>

这种方式是直接用sql将多对一的结果查询出来 需存到teacher对象中的参数映射在association标签中定义

我个人认为第二种更好一点 因为遇到复杂的sql更直观

 

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一对多（一个老师对一个学生  即teacher实体类中存了student集合对象）

Mybatis一对多实现方式1：

@Data
public class Teacher {
    private int id;
    private String name;
    //一个老师多个学生
    private List<Student> students;
}

@Data
public class Student {
    private int id;
    private String name;
    //tid对应teacher的id
    private int tid;
}

  <resultMap id="TeacherStudent" type="bean.Teacher">
        <result  property="name" column="tname"/>
    //collection标签同上例类似 但它是存student集合的 collection就是集合很好记
        <collection property="students" ofType="bean.Student">
            <result property="id" column="sid" />
            <result property="name" column="sname" />
            <result property="tid" column="tid" />
        </collection>
    </resultMap>

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

一对多例子同上类似 只不过将查询多对一中存单个对象变为存集合 标签collection就是集合