左连接查询

场景：查询老师，并且把关联的学生也查出来。

Teacher.java 映射文件：

@Getter@Setter@ToString
public class Teacher {private Integer teacher_id;private String teacher_name;List<Student> students = new ArrayList<>(); // 一个老师对应多个学生
}

创建一个 TeacherMapper 接口文件，代码如下：

public interface TeacherMapper {/*查询老师，并且把关联的学生也查出来*/public List<Teacher> getAllTeachers();
}

创建一个 TeacherMapper 的映射文件：

<mapper namespace="com.itlike.mapper.TeacherMapper"><select id="getAllTeachers" resultMap="teacherMap">SELECT * FROM `teacher` as tLEFT JOIN `stu_teacher_rel` as rON t.teacher_id = r.teacher_idLEFT JOIN `student` as sON s.stu_id = r.stu_id;</select><resultMap id="teacherMap" type="teacher"><id column="teacher_id" property="teacher_id"/><result column="teacher_id" property="teacher_id"/><collection property="students" javaType="list" ofType="student"><id column="stu_id" property="stu_id"/><result column="stu_name" property="stu_name"/></collection></resultMap></mapper>

测试类中：

public void test(){SqlSession sqlSession = MybatisUtils.openSession();TeacherMapper teacherMapper = sqlSession.getMapper(TeacherMapper.class);List<Teacher> allTeachers = teacherMapper.getAllTeachers();for (Teacher teacher : allTeachers) {System.out.println(teacher);}}

运行效果：与数据库查询结果相同。