Mybatis注解实现多表关联查询

2020.3.16

---

Mybatis注解实现多表关联查询

1、Demo描述

1）功能描述：

​ 学生提出问题，多个医生可以回答一个学生的问题。医生回答问题时，页面上不仅要显示问题的基本信息，也要显示该问题必要的学生信息。

2）E-R图：

​ 由于学生和问题是1对多的关系，医生和问题是多对1的关系，又因为问题还与其他的属性捆绑，个人觉得使用中间表来显示学生-问题-医生三者之间的关系会比较清晰

2、Entity类

备注：省略getter,setter,toString

public class Student {

    private String id;
    private String username;
    private String password;
    private Integer gender;
    private Integer age;
    private String tel;
    private String email;
    //学生 - 问题：1对多
    private List<Question> questionList;

    ...
｝

public class Question implements Serializable{

    private String id;
    private String createTime;
    private String updateTime;
    private String content;
    private Integer viewCount;
    private Integer likes;
    private boolean anonymous;
    private boolean status;
    private Student student;
    //问题 - 医生：多对1
    private List<Doctor> doctors;
  
    ...
}

public class Doctor {

    private String id;
    private String username;
    private String password;
    private Integer gender;
    private Integer age;
    private Integer teachYear;
    private String graduatedSchool;
    private String tel;
    private String email;
  
    ...
}

3、数据库表设计

1）student表

2）question表

3）doctor表

4）中间表及其外键约束

4、方法实现

0）备注：

• 方法功能描述：通过对问题集合的查询，该方法会一次性地将每个问题对应的学生id，username和医生id封装在问题的student，doctors对象中，并返回问题集合。
• @Result：若查询时，数据库表字段和实体类字段命名不一致，需要手动配置映射关系
• @one：1对1的关联查询，配置result时，相当于将子查询的结果与实体类的字段进行匹配
• @many：1对多的关联查询，配置result时，相当于将子查询的结果与实体类的字段进行匹配
• @Select()，@Insert()等注解简化mapper.xml的配置

1）Mapper中的方法及其实现

• getQuestionsWithStuAndDoctors() 主查询

 @Results({
            @Result(column = "id",property = "id"),
            @Result(column = "id",property = "student",one = @One(select = "com.wangxiaoxi.mheal.mapper.StudentMapper.getStuByQuesId",fetchType=FetchType.DEFAULT)),
            @Result(column = "id",property = "doctors",many = @Many(select = "com.wangxiaoxi.mheal.mapper.DoctorMapper.getDoctorsByQuesId",fetchType=FetchType.DEFAULT)),
    })
 @Select("select * from question order by updateTime")
 public List<Question> getQuestionsWithStuAndDoctors();

• getStuByQuesId() 子查询

  • 将结果封装到student字段中

    @Results({
           @Result(column = "s_id",property = "id"),
           @Result(column = "s_id",property = "username",one = @One(select = "com.wangxiaoxi.mheal.mapper.StudentMapper.getUsernameById",fetchType = FetchType.DEFAULT))
    })
    @Select("select distinct s_id from askAndAns where q_id = #{q_id}")
    Student getStuByQuesId(String q_id);
    

  • getUsernameById() 子子查询，将结果封装到student对象的username字段中

    @Select("select username from student where id = #{s_id}")
    public String getUsernameById(String s_id);
    

• getDoctorsByQuesId() 子查询，将结果封装到doctors字段中

@Results({
     @Result(column = "d_id",property = "id")
})
@Select("select d_id from askAndAns where q_id = #{q_id}")
List<Doctor> getDoctorsByQuesId(String q_id);

2）测试结果

@Test
void contextLoads() {
	/**------------------------------中间表查询测试------------------------------------*/
	List<Question> questions = questionMapper.getQuestionsWithStuAndDoctors();
	for(Question q : questions){
		System.out.println(q);
	}
}

2020-03-16 21:13:41.922 DEBUG 8388 --- [           main] c.w.m.m.Q.getQuestionsWithStuAndDoctors  : ==>  Preparing: select * from question order by updateTime 
2020-03-16 21:13:41.922 DEBUG 8388 --- [           main] c.w.m.m.Q.getQuestionsWithStuAndDoctors  : ==> Parameters: 
2020-03-16 21:13:41.938 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getStuByQuesId 

//子查询1
: ====>  Preparing: select distinct s_id from askAndAns where q_id = ? 

2020-03-16 21:13:41.939 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getStuByQuesId     : ====> Parameters: f2882808-4db0-4d3c-bc81-4f8e484aef2b(String)
2020-03-16 21:13:41.951 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getUsernameById

//子子查询1
: ======>  Preparing: select username from student where id = ? 

2020-03-16 21:13:41.953 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getUsernameById    : ======> Parameters: 1614010820(String)
2020-03-16 21:13:41.962 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getUsernameById    : <======      Total: 1
2020-03-16 21:13:41.963 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getStuByQuesId     : <====      Total: 1
2020-03-16 21:13:41.963 DEBUG 8388 --- [           main] c.w.m.m.DoctorMapper.getDoctorsByQuesId 

//子查询2
: ====>  Preparing: select d_id from askAndAns where q_id = ? 

2020-03-16 21:13:41.964 DEBUG 8388 --- [           main] c.w.m.m.DoctorMapper.getDoctorsByQuesId  : ====> Parameters: f2882808-4db0-4d3c-bc81-4f8e484aef2b(String)
2020-03-16 21:13:41.969 DEBUG 8388 --- [           main] c.w.m.m.DoctorMapper.getDoctorsByQuesId  : <====      Total: 2
2020-03-16 21:13:41.970 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getStuByQuesId     : ====>  Preparing: select distinct s_id from askAndAns where q_id = ? 
2020-03-16 21:13:41.971 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getStuByQuesId     : ====> Parameters: f2882808-4db0-4d3c-bc81-4f8e484aef3b(String)
2020-03-16 21:13:41.974 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getUsernameById    : ======>  Preparing: select username from student where id = ? 
2020-03-16 21:13:41.975 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getUsernameById    : ======> Parameters: 1614010821(String)
2020-03-16 21:13:41.977 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getUsernameById    : <======      Total: 1
2020-03-16 21:13:41.978 DEBUG 8388 --- [           main] c.w.m.m.StudentMapper.getStuByQuesId     : <====      Total: 1
2020-03-16 21:13:41.979 DEBUG 8388 --- [           main] c.w.m.m.DoctorMapper.getDoctorsByQuesId  : ====>  Preparing: select d_id from askAndAns where q_id = ? 
2020-03-16 21:13:41.979 DEBUG 8388 --- [           main] c.w.m.m.DoctorMapper.getDoctorsByQuesId  : ====> Parameters: f2882808-4db0-4d3c-bc81-4f8e484aef3b(String)
2020-03-16 21:13:41.983 DEBUG 8388 --- [           main] c.w.m.m.DoctorMapper.getDoctorsByQuesId  : <====      Total: 1
2020-03-16 21:13:41.984 DEBUG 8388 --- [           main] c.w.m.m.Q.getQuestionsWithStuAndDoctors  : <==      Total: 2

//总查询
Question{id='f2882808-4db0-4d3c-bc81-4f8e484aef2b', createTime='2020-03-07 10:27:19', updateTime='2020-02-26 10:27:19', content='医生，最近压力大，请问如何才能正确排解压力 ', viewCount=0, likes=0, anonymous=false, status=false, student=Student{id='1614010820', username='王XX', password='null', gender=null, age=null, tel='null', email='null', questionList=null}, doctorList=[Doctor{id='161401080801', username='null', password='null', gender=null, age=null, teachYear=null, graduatedSchool='null', tel='null', email='null'}, Doctor{id='161401080802', username='null', password='null', gender=null, age=null, teachYear=null, graduatedSchool='null', tel='null', email='null'}]}

Question{id='f2882808-4db0-4d3c-bc81-4f8e484aef3b', createTime='2020-03-07 10:27:19', updateTime='2020-02-26 10:27:19', content='医生，最近压力大，请问如何才能正确排解压力 ', viewCount=0, likes=0, anonymous=false, status=false, student=Student{id='1614010821', username='王小希', password='null', gender=null, age=null, tel='null', email='null', questionList=null}, doctorList=[Doctor{id='161401080802', username='null', password='null', gender=null, age=null, teachYear=null, graduatedSchool='null', tel='null', email='null'}]}

2020-03-16 21:13:42.087  INFO 8388 --- [extShutdownHook] o.s.s.concurrent.ThreadPoolTaskExecutor  : Shutting down ExecutorService 'applicationTaskExecutor'
2020-03-16 21:13:42.122  INFO 8388 --- [extShutdownHook] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown initiated...
2020-03-16 21:13:42.171  INFO 8388 --- [extShutdownHook] com.zaxxer.hikari.HikariDataSource       : HikariPool-1 - Shutdown completed.

Process finished with exit code 0

5、注意事项

• 注意sql查询结果是否唯一，若查询结果出现重复值，用distinct关键字，查出一个值即可，否则会出现returned more than one row, where no more than one was expected.异常
• 在执行子查询时，发现**@one和@many处理效果相同**，也许这么写可以提高代码的可读性
• 注解的作用可以提高代码的可读性，增强代码功能，减少代码的编写量。column为子查询的参数，property为子查询要封装的对象@Result，@one，@many在查询中嵌套子查询来增强主查询的功能。

6、参考文档

1、MyBatis学习总结（十）—基于注解的多表查询（一对一，一对多，多对多）

2、[mybatis 注解@Results、@Result、@ResultMap、@One的使用

3、returned more than one row异常