Mybatis 复杂查询多对一 和 一对多

多对一的处理：（尽量使用方法一）

@Data
public class Student {
    private int id;
    private String name;
    //学生需要关联一个老师
    private Teacher teacher;
}

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

 

public interface StudentMapper {

    List<Student>  getStudent();

    List<Student> getStudent2();
}

 

<!--方法一 根据结构查询  联表 -->
    <select id="getStudent2" 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">
　　　　　　#property 属性名  column字段名 起了别名写别名
　　　　　　#简单字段可以直接这些写
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
　　　　#复杂字段  返回是对象使用association
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

 <!-- 方法二  子查询   嵌套-->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student ;
    </select>

    <resultMap id="StudentTeacher" type="Student">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        <!--          复杂的属性，我们需要单独处理   对象：association  集合：collection-->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>

    <select id="getTeacher" resultType="Teacher">
        select * from teacher where id = #{tid};
    </select>

一对多的处理：（尽量使用方法一）

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


@Data
public class Teacher {
    private int id;
    private String name;    private List<Student> students;
}

 

public interface TeacherMapper {
    //获取指定老师下的学生及老师信息
    Teacher getTeacher(@Param("tid")int id);

    Teacher getTeacher2(@Param("tid") int id);
}

 

<!-- 方法一 根据结构查询  -->
    <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 = #{tid}
    </select>
    <resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"/>
        <result property="name" column="tname"/>
        <!-- 集合中的泛型信息，我们使用ofType获取 -->
        <collection property="students" ofType="Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
            <result property="tid" column="tid"/>
        </collection>
    </resultMap>

<!--  方法二   子查询  -->
    <select id="getTeacher2" resultMap="TeacherStudent2">
        select *
        from mybatis.teacher where  id=#{tid};
    </select>
    <resultMap id="TeacherStudent2" type="Teacher">
        <collection property="students" javaType="Arraylist" ofType="Student" select="getStudentByTeacherId" column="id"/>
    </resultMap>
    <select id="getStudentByTeacherId" resultType="Student">
        select * from mybatis.student where  tid=#{tid}
    </select>