MyBatis 查询时属性中一对多的处理（一条数据对应多条数据）

多对一处理：https://blog.csdn.net/weixin_44953227/article/details/112786245

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前期准备

数据表

CREATE TABLE `teacher`(
  id INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  PRIMARY KEY (id)
) ENGINE=INNODB DEFAULT CHARSET=utf8;


INSERT INTO `teacher`(id,`name`) VALUES(1,'大师');

CREATE TABLE `student`(
  id INT(10) NOT NULL,
  `name` VARCHAR(30) DEFAULT NULL,
  `tid` INT(10) DEFAULT NULL,
  PRIMARY KEY(id),
  KEY `fktid` (`tid`),
  CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`)
) ENGINE=INNODB DEFAULT CHARSET=utf8;

INSERT INTO student(`id`,`name`,`tid`) VALUES(1,'小明',1);
INSERT INTO student(`id`,`name`,`tid`) VALUES(2,'小红',1);
INSERT INTO student(`id`,`name`,`tid`) VALUES(3,'小张',1);
INSERT INTO student(`id`,`name`,`tid`) VALUES(4,'小李',1);
INSERT INTO student(`id`,`name`,`tid`) VALUES(5,'小王',1);

Student 实体类

public class Student {
     
    private int id;
    private String name;

    private int tid;
}

Teacher 实体类

public class Teacher {
     
    private int id;
    private String name;

    private List<Student> students;
}

接口

public interface TeacherMapper {
     
    // 查询嵌套处理 - 子查询
    Teacher getTeacherById(int id);

    // 结果嵌套处理
    Teacher getTeacherResultById(int id);
}

按照查询嵌套处理

• collection：处理集合
  • property：实体类中属性字段
  • column：查询结果中需要传递给子查询的字段
  • javaType：指定属性的类型
  • ofType：指定集合中泛型
  • select：子查询SQL

<mapper namespace="com.pro.dao.TeacherMapper">
    
    <resultMap id="TeacherMap" type="com.pro.pojo.Teacher">
        <result property="id" column="id"/>
        <collection property="students" column="id" javaType="List" ofType="com.pro.pojo.Student" select="getStudentById"/>
    resultMap>

    <select id="getTeacherById" resultMap="TeacherMap">
        select * from teacher where id = #{id}
    select>

    
    <select id="getStudentById" resultType="com.pro.pojo.Student">
        select * from student where tid = #{id}
    select>
mapper>

按照结果嵌套处理

• collection：处理集合
  • javaType：指定属性的类型
  • ofType：指定集合中泛型

<mapper namespace="com.pro.dao.TeacherMapper">
    
    <resultMap id="TeacherResult" type="com.pro.pojo.Teacher">
        <result property="id" column="id"/>
        <result property="name" column="name"/>
        
        <collection property="students" javaType="List" ofType="com.pro.pojo.Student">
            <result property="id" column="sid"/>
            <result property="name" column="sname"/>
        collection>
    resultMap>

    <select id="getTeacherResultById" resultMap="TeacherResult">
        select t.*, s.id sid, s.name sname  from teacher t, student s where t.id = #{id} and t.id = s.tid
    select>
mapper>

小结

1. 关联 - association 处理多对一
2. 集合 - collection 处理一对多
3. javaType & ofType
   1. javaType： 用来指定实体类中属性的类型
   2. ogType： 指定集合中的类型，泛型中的约束类型