在 sql 中我们就学习过多对一和一对多的处理，现在我们要在 MyBatis 中实现对多对一和一对多的处理。

很经典的就是学生和老师之间的关系

对于学生而言：多个学生关联一个老师（多对一）

对于老师而言：一个老师教育多个学生（一对多）

创建老师表和学生表

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, '张老师')

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', '小李', '1');
INSERT INTO `student`(`id`, `name`, `tid`) VALUES ('4', '小王', '1');
INSERT INTO `student`(`id`, `name`, `tid`) VALUES ('5', '小军', '1');

测试环境搭建

实体类 Teacher 和 Student

@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;
}

Mapper 接口

public interface StudentMapper {
}

public interface TeacherMapper {
    @Select("select * from teacher where id = #{tid}")
    Teacher getTeacher(@Param("tid") int id);
}

对应 Mapper.xml 文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zhouyue.dao.StudentMapper">

</mapper>

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.zhouyue.dao.TeacherMapper">

</mapper>

核心配置文件配置 mapper 接口或文件

<mappers>
    <mapper class="com.zhouyue.dao.TeacherMapper"></mapper>
    <mapper class="com.zhouyue.dao.StudentMapper"></mapper>
</mappers>

测试查询

    @Test
    public void test1(){
        SqlSession sqlSession = MyBatisUtils.getSqlSession();
        TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
        Teacher teacher = mapper.getTeacher(1);
        System.out.println(teacher);
        sqlSession.close();
    }

多对一

现在有一个稍微复杂的需求：要求查询出所有学生以及对应的老师信息

直接使用 sql 查询：

按照查询嵌套处理：

<!--
    思路:
    1. 查询所有学生信息
    2. 根据查询出来的学生的tid，寻找对应的老师
    -->
    <select id="getStudent" resultMap="StudentTeacher">
        select * from student;
    </select>
    <resultMap id="StudentTeacher" type="student">
        <result property="id" column="id"></result>
        <result property="name" column="name"></result>
    <!--
        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"></association>

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

按照结果嵌套处理

<!--  按照结果嵌套处理  -->
    <select id="getStudent" resultMap="StudentTeacher">
        select s.id sid, s.name sname, t.name tname
        from student s, teacher t
        where s.tid = t.id;
    </select>
    <resultMap id="StudentTeacher" 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

将上面项目稍微修改

student类

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

teacher类

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

我们现在要查询指定老师及其所教学生的信息

按照结果嵌套处理

<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 column="tid" property="id"></result>
        <result column="tname" property="name"></result>
        <!--
            javaType= 是指定属性的类型
            集合的话使用 ofType
        -->
        <collection property="students" ofType="student">
            <result property="id" column="sid"></result>
            <result property="name" column="sname"></result>
            <result property="tid" column="tid"></result>
        </collection>
    </resultMap>

按照查询嵌套处理

<select id="getTeacher" resultMap="TeacherStudent">
        select * from teacher where id = #{tid};
    </select>
    <resultMap id="TeacherStudent" type="teacher">
        <result property="id" column="id"></result>
        <result property="name" column="name"></result>
        <collection property="students" column="id" javaType="ArrayList" ofType="Student" select="getStudentByTid"></collection>
    </resultMap>
    <select id="getStudentByTid" resultType="Student">
        select * from student where tid = #{tid}
    </select>