1、复杂环境搭建

这个环境搭建跟我mybatis第一篇文章的环境搭建没有什么太大的区别，只不过这里会用到一些我前几篇说到的知识点，如果没看过我之前的文章，可以去看我前几篇的文章。

步骤

工具类（和之前的一样，不变）；
创建实体类（属性名都要对应数据库列名）；
创建接口；
在resources包下建立和接口对应的包和xml配置文件；
在mybatis-config.xml核心配置文件中绑定所创建的接口。
设置别名

环境搭建好后就可以接下来的操作了。

2、多对一处理

多对一：例如很多学生都有同一个老师，我们从学生方面去查询学生和老师。

2.1、实体类

实体类——学生

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Student {
    private int id;
    private String name;
    //老师的id是要从老师的表去查询，所以给返回老师的对象
    private Teacher teacher;
}

实体类——老师

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

2.2、按照查询嵌套处理

StudentMapper接口

public interface StudentMapper {
    List<Student> selectStudentAndTeacher();
}

StudentMapper.xml配置文件

<mapper namespace="com.ZXF.dao.StudentMapper">
<!--    通过查询嵌套=========================================-->
<select id="selectStudentAndTeacher" resultMap="StudentTeacher">
    select * from student;
</select>
    <resultMap id="StudentTeacher" type="Student">
        <result column="id" property="id"/>
        <result column="name" property="name"/>
    <!--  复杂的属性，单独处理，如果要处理的是：
            对象：用association
            集合：用collection
        javaType：因为我们要处理的对象是一个类，所以我们把这个类传递进去
        -->
        <association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
    </resultMap>
<select id="getTeacher" resultType="Teacher">
    <!--单个参数时，#{id}这个参数名可以随意写，但为了命名规范和方便排错，建议一致 -->
    select * from teacher where id=#{id};
</select>

测试

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

测试结果
在这里插入图片描述

2.3、按照结果嵌套处理

StudentMapper接口

public interface StudentMapper {
    List<Student> selectStudentAndTeacher2();
}

StudentMapper.xml配置文件

<!--    通过结果嵌套=========================================-->
<select id="selectStudentAndTeacher2" 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 column="sid" property="id"/>
    <result column="sname" property="name"/>
    <association property="teacher" javaType="Teacher">
        <result column="tname" property="name"/>
    </association>
</resultMap>

测试

@Test
public  void  selectStudentAndTeacher2(){
    SqlSession sqlSession = MybatisUtil.getSqlSession();
    StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
    List<Student> students2 = mapper.selectStudentAndTeacher2();
    for (Student student : students2) {
        System.out.println(student);
    }
    sqlSession.close();
}

测试结果
在这里插入图片描述

3、一对多处理

一对多：例如一个老师教好多个学生。我们从老师方面去查询学生和老师。

3.1、实体类

实体类——学生

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

实体类——老师

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Teacher {
    private int id;
    private String name;
    //这里是从老师方向查学生，返回集合
    private List<Student> students;
}

3.2、按结果嵌套查询

TeacherMapper接口

public interface TeacherMapper {
    //查询老师和学生
    List<Teacher> getTeacher(int id);
}

TeacherMapper.xml配置文件

<select id="getTeacher" resultMap="TeacherStudent">
    select s.id sid,s.name sname,t.name tname,t.id tid
    from student s,teacher t
    where t.id=#{tid}
</select>
<resultMap id="TeacherStudent" type="Teacher">
    <result property="id" column="tid"/>
    <result property="name" column="tname"/>
    <!--        javaType ： 指定属性的类型
            ofType ： 集合中的泛型信息
                -->
    <collection property="students" ofType="Student">
        <result column="sid" property="id"/>
        <result column="sname" property="name"/>
        <result column="tid" property="tid"/>
    </collection>
</resultMap>

测试

@Test
    public void getTeacherStudent(){
    SqlSession sqlSession = MybatisUtil.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    List<Teacher> teacher = mapper.getTeacher(1);
    for (Teacher teacher1 : teacher) {
        System.out.println(teacher1);
    }
    sqlSession.close();
}

测试结果
在这里插入图片描述

3.3、按查询嵌套处理

TeacherMapper接口

public interface TeacherMapper {
    //查询老师和学生
    List<Teacher> getTeacher2(int id);
}

TeacherMapper.xml配置文件

<select id="getTeacher2" resultMap="TeacherStudent2">
    select id,name from teacher where id=#{id}
</select>
<resultMap id="TeacherStudent2" type="Teacher">
    <collection property="students" javaType="ArrayList" ofType="Student" select="getStudentById" column="id">
    </collection>
</resultMap>
<select id="getStudentById" resultType="Student">
    select * from student where tid=#{id}
</select>
</

测试

@Test
    public void getTeacherStudent2(){
    SqlSession sqlSession = MybatisUtil.getSqlSession();
    TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
    List<Teacher> teacher2 = mapper.getTeacher2(1);
    for (Teacher teacher : teacher2) {
        System.out.println(teacher);
    }
    sqlSession.close();
}