Mybatis——多对一的处理

测试环境搭建

1.新建实体类

2.建立Mapper接口

3.建立Mapper.xml文件

4.在核心配置文件中绑定注册Mapper接口或者文件

5.测试

按照查询嵌套处理

实体类

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

Mapper接口

//查询所有的学生信息以及对应的老师信息
List<Student> getStudents();

Mapper.xml文件

<select id="getStudents" resultMap="StudentTeacher">
    select * from student
</select>
​
<select id="getTeacherById" resultType="Teacher">
    select * from teacher where id = #{id}
</select>
​
<resultMap id="StudentTeacher" type="Student">
    <result property="id" column="id"/>
    <result property="name" column="name"/>
    <!--复杂的语句单独处理，属性是Teacher类，通过tid列，利用getTeacherById的select进行嵌套查询-->
    <association property="teacher" column="tid" javaType="Teacher" select="getTeacherById"/>
</resultMap>

相当于子查询

按照结果嵌套处理

<select id="getStudents2" resultMap="StudentTeacher2">
    select s.id sid,s.name sname,t.id tid,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 property="name" column="sname"/>
    <association property="teacher" javaType="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
    </association>
</resultMap>

相当于联表查询