多对一

多个学生对于一个老师

环境搭建

导入lombok

 <dependencies>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.22</version>
        </dependency>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>RELEASE</version>
            <scope>compile</scope>
        </dependency>
    </dependencies>

新建实体类Teacher,Student

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

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

建立Mapper接口

public interface StudentMapper {
    //查询所有学生信息，以及对于老师的信息
    public List<Student> getStudent();
    public List<Student> getStudent2();
}

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

建立Mapper.xml文件(StudentMapper.xml)

<?xml version="1.0" encoding="UTF8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.weirdo.dao.StudentMapper">

</mapper>

在核心配置文件中绑定注册我们的Mapper接口或者文件【方式很多，随心选】

<!--绑定接口-->
    <mappers>
        <mapper class="com.weirdo.dao.TeacherMapper"/>
        <mapper class="com.weirdo.dao.StudentMapper"/>
    </mappers>

测试查询是否能够成功

按照查询嵌套处理

 <!--思路：
        1.查询所有的学生信息
        2.根据查询出来的学生tid，寻找对应的老师!!!子查询！！！
    -->
     <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=#{id};
    </select>

   @Test
    public void testStudent(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> studentlist = mapper.getStudent();
        for (Student student : studentlist) {
            System.out.println(student);
        }
        sqlSession.close();
    }

按照结果嵌套处理

<!--按照结果嵌套处理-->
    <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">
        <result property="id" column="sid"/>
        <result property="name" column="sname"/>
        <association property="teacher" javaType="Teacher">
            <result property="name" column="tname"/>
        </association>
    </resultMap>

@Test
    public void testStudent2(){
        SqlSession sqlSession = MybatisUtils.getSqlSession();
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        List<Student> studentlist = mapper.getStudent2();
        for (Student student : studentlist) {
            System.out.println(student);
        }
        sqlSession.close();
    }

回顾Mysql多对一查询方式:

子查询（按照查询嵌套）
联表查询（按照结果嵌套）

一对多

按照查询嵌套处理

<!--按照查询嵌套处理-->
    <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>

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

结果输出

Teacher(id=1, name=张老师,students=[Student(id=1, name=小明, tid=1), Student(id=2, name=小红, tid=1), Student(id=3, name=小兰, tid=1), Student(id=4, name=小黑, tid=1), Student(id=5, name=小白, tid=1)])

按照结果嵌套处理

<!--    按照结果嵌套查询-->
       <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"/>
        <!--复杂的属性，我们需要单独处理 对象：association  集合：collection
        javatype 指定属性的类型
        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>