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最简单的 MyBatis Plus 的多表联接、分页查询实现方法

2019-07-03 10:53 579 查看

转载url:
https://blog.csdn.net/larger5/article/details/82082819

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<h1><a name="t0"></a><a id="_0"></a>一、前言</h1>

最近在加强 ITAEM 团队的一个 app 项目——学生教师学习交流平台
人员组成：安卓 + 前端 + 后台
后台 DAO 层借鉴了华工其他软件开发团队，使用了新颖强大的

MyBatisPlus

框架，里边有一个类似百度贴吧的发帖子的功能：
![这里写图片描述](https://img-blog.csdn.net/20180826164333911?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhcmdlcjU=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70)
而如果设计表，应为

帖子表 t_post
- id
- title 标题
- content 内容
- xx
- user_id 用户外键

用户表 t_user id
name 帖子发起者名字
xx

示例图中红色框中的内容为

t_user

表的字段

name

，
而要实现上面显示帖子，就要用到关联查询了，而且帖子很多，必须用分页查询，

那么，怎么通过 MyBatisPlus 来实现关联、分页查询呢？很简单，往下看。

二、需求、数据库表设计

这是个人 app 项目中 v1.0 版本的部分表。
![这里写图片描述](https://img-blog.csdn.net/20180826165727789?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhcmdlcjU=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70)
需求：显示帖子

要帖子基本内容如时间、帖子内容等，即
```
t_question
```
表的内容全部要，
同时还要发帖子的人名字，即
```
t_student
```
的字段
```
name
```

三、代码结构

为了写这篇文章，抽取了该 app 项目中的部分代码，彼此相互关系如下图
![这里写图片描述](https://img-blog.csdn.net/20180826171008366?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhcmdlcjU=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70)

四、代码实现

1、代码已经放到 github 上了，若对本文的代码有疑问可以去 github 上查看详情：
https://github.com/larger5/MyBatisPlus_page_tables.git

2、

entity、mapper、service、controller

使用了 MyBatisPlus 的代码生成器，自动生成大部分基础的代码，操作方法见之前的文章：
在 SpringBoot 中引入 MyBatisPlus 之常规操作

1.实体

① Question

// import 省略

@TableName(“t_question”)
public class Question implements Serializable {

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> serialVersionUID <span class="token operator">=</span> <span class="token number">1</span>L<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问答主键id"</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@TableId</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"id"</span><span class="token punctuation">,</span> type <span class="token operator">=</span> IdType<span class="token punctuation">.</span>AUTO<span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer id<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生外键id"</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@TableField</span><span class="token punctuation">(</span><span class="token string">"student_id"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer studentId<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问题内容"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String content<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问题发布时间，发布的时候后台自动生成"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Date date<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问题悬赏的积分"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer value<span class="token punctuation">;</span>

<span class="token comment">// getter、setter 省略</span>

}

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② Student

// import 省略

@TableName(“t_student”)
public class Student implements Serializable {

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> serialVersionUID <span class="token operator">=</span> <span class="token number">1</span>L<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生主键id"</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@TableId</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"id"</span><span class="token punctuation">,</span> type <span class="token operator">=</span> IdType<span class="token punctuation">.</span>AUTO<span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer id<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生名称"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String name<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生密码"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String password<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生积分数"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer points<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生邮件地址"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String email<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生手机号码"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String phone<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生学号"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String num<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生真实姓名"</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@TableField</span><span class="token punctuation">(</span><span class="token string">"true_name"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String trueName<span class="token punctuation">;</span>

<span class="token comment">// getter、setter 省略</span>

}

2.mapper

① StudentMapper

// import 省略public interface StudentMapper extends BaseMapper<Student> {
}

[/code]

② QuestionMapper

// import 省略public interface QuestionMapper extends BaseMapper<Question> {
/**
*
* @param page 翻页对象，可以作为 xml 参数直接使用，传递参数 Page 即自动分页
* @return
*/
@Select("SELECT t_question.*,t_student.`name` FROM t_question,t_student WHERE t_question.student_id=t_student.id")
List<QuestionStudentVO> getQuestionStudent(Pagination page);

}

3、service

① StudentService

// import 省略public interface StudentService extends IService<Student> {
}

[/code]

② QuestionService

// import 省略public interface QuestionService extends IService<Question> {

Page<span class="token generics function"><span class="token punctuation">&lt;</span>QuestionStudentVO<span class="token punctuation">&gt;</span></span> <span class="token function">getQuestionStudent</span><span class="token punctuation">(</span>Page<span class="token generics function"><span class="token punctuation">&lt;</span>QuestionStudentVO<span class="token punctuation">&gt;</span></span> page<span class="token punctuation">)</span><span class="token punctuation">;</span>

[/code]

}

4、serviceImpl

① StudentServiceImpl

// import 省略@Service
public class StudentServiceImpl extends ServiceImpl<StudentMapper, Student> implements StudentService {

}

② QuestionServiceImpl

// 省略 import

@Service
public class QuestionServiceImpl extends ServiceImpl<QuestionMapper, Question> implements QuestionService {

<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> Page<span class="token generics function"><span class="token punctuation">&lt;</span>QuestionStudentVO<span class="token punctuation">&gt;</span></span> <span class="token function">getQuestionStudent</span><span class="token punctuation">(</span>Page<span class="token generics function"><span class="token punctuation">&lt;</span>QuestionStudentVO<span class="token punctuation">&gt;</span></span> page<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> page<span class="token punctuation">.</span><span class="token function">setRecords</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>baseMapper<span class="token punctuation">.</span><span class="token function">getQuestionStudent</span><span class="token punctuation">(</span>page<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

5、controller

// 省略 import

@RestController
@RequestMapping("/common")
@EnableSwagger2
public class CommonController {

<span class="token annotation punctuation">@Autowired</span>
QuestionService questionService<span class="token punctuation">;</span>

<span class="token annotation punctuation">@Autowired</span>
StudentService studentService<span class="token punctuation">;</span>

<span class="token annotation punctuation">@GetMapping</span><span class="token punctuation">(</span><span class="token string">"/getAllQuestionByPage/{page}/{size}"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> Map<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">,</span> Object<span class="token punctuation">&gt;</span></span> <span class="token function">getAllQuestionByPage</span><span class="token punctuation">(</span><span class="token annotation punctuation">@PathVariable</span> Integer page<span class="token punctuation">,</span> <span class="token annotation punctuation">@PathVariable</span> Integer size<span class="token punctuation">)</span> <span class="token punctuation">{</span>
Map<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">,</span> Object<span class="token punctuation">&gt;</span></span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token operator">&lt;</span><span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Page<span class="token generics function"><span class="token punctuation">&lt;</span>Question<span class="token punctuation">&gt;</span></span> questionPage <span class="token operator">=</span> questionService<span class="token punctuation">.</span><span class="token function">selectPage</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Page</span><span class="token operator">&lt;</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>page<span class="token punctuation">,</span> size<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>questionPage<span class="token punctuation">.</span><span class="token function">getRecords</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"code"</span><span class="token punctuation">,</span> <span class="token number">400</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"code"</span><span class="token punctuation">,</span> <span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"data"</span><span class="token punctuation">,</span> questionPage<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> map<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@GetMapping</span><span class="token punctuation">(</span><span class="token string">"/getAllQuestionWithStudentByPage/{page}/{size}"</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> Map<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">,</span> Object<span class="token punctuation">&gt;</span></span> <span class="token function">getAllQuestionWithStudentByPage</span><span class="token punctuation">(</span><span class="token annotation punctuation">@PathVariable</span> Integer page<span class="token punctuation">,</span> <span class="token annotation punctuation">@PathVariable</span> Integer size<span class="token punctuation">)</span> <span class="token punctuation">{</span>
Map<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">,</span> Object<span class="token punctuation">&gt;</span></span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token operator">&lt;</span><span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
Page<span class="token generics function"><span class="token punctuation">&lt;</span>QuestionStudentVO<span class="token punctuation">&gt;</span></span> questionStudent <span class="token operator">=</span> questionService<span class="token punctuation">.</span><span class="token function">getQuestionStudent</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Page</span><span class="token operator">&lt;</span><span class="token operator">&gt;</span><span class="token punctuation">(</span>page<span class="token punctuation">,</span> size<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>questionStudent<span class="token punctuation">.</span><span class="token function">getRecords</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"code"</span><span class="token punctuation">,</span> <span class="token number">400</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"code"</span><span class="token punctuation">,</span> <span class="token number">200</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"data"</span><span class="token punctuation">,</span> questionStudent<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> map<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

6、MyBatisPlus 配置

// 省略 import

@EnableTransactionManagement
@Configuration
@MapperScan(“com.cun.app.mapper”)
public class MybatisPlusConfig {

<span class="token comment">/**
* 分页插件
*/</span>
<span class="token annotation punctuation">@Bean</span>
<span class="token keyword">public</span> PaginationInterceptor <span class="token function">paginationInterceptor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token keyword">new</span> <span class="token class-name">PaginationInterceptor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">/**
* 打印 sql
*/</span>
<span class="token annotation punctuation">@Bean</span>
<span class="token keyword">public</span> PerformanceInterceptor <span class="token function">performanceInterceptor</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
PerformanceInterceptor performanceInterceptor <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">PerformanceInterceptor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//格式化sql语句</span>
Properties properties <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Properties</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
properties<span class="token punctuation">.</span><span class="token function">setProperty</span><span class="token punctuation">(</span><span class="token string">"format"</span><span class="token punctuation">,</span> <span class="token string">"true"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
performanceInterceptor<span class="token punctuation">.</span><span class="token function">setProperties</span><span class="token punctuation">(</span>properties<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> performanceInterceptor<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

7、关联查询 VO 对象

// import 省略

public class QuestionStudentVO implements Serializable {

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问答主键id"</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@TableId</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"id"</span><span class="token punctuation">,</span> type <span class="token operator">=</span> IdType<span class="token punctuation">.</span>AUTO<span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer id<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"学生外键id"</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@TableField</span><span class="token punctuation">(</span><span class="token string">"student_id"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer studentId<span class="token punctuation">;</span>

<span class="token keyword">private</span> String name<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问题内容"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> String content<span class="token punctuation">;</span>

<span class="token annotat
4000
ion punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问题发布时间，发布的时候后台自动生成"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Date date<span class="token punctuation">;</span>

<span class="token annotation punctuation">@ApiModelProperty</span><span class="token punctuation">(</span>value <span class="token operator">=</span> <span class="token string">"问题悬赏的积分"</span><span class="token punctuation">)</span>
<span class="token keyword">private</span> Integer value<span class="token punctuation">;</span>

<span class="token comment">// getter、setter 省略</span>

五、测试接口

![这里写图片描述](https://img-blog.csdn.net/20180826180214880?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhcmdlcjU=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70)

1、没有关联的分页查询接口

http://localhost/common/getAllQuestionByPage/1/2

① json 输出

{
"code": 200,
"data": {
"total": 10,
"size": 2,
"current": 1,
"records": [
{
"id": 1,
"studentId": 3,
"content": "唐代，渝州城里，有一个性格开朗、乐观的小伙子，名叫景天。",
"date": 1534497561000,
"value": 5
},
{
"id": 2,
"studentId": 1,
"content": "雪见从小父母双亡，由爷爷唐坤抚养成人。",
"date": 1533201716000,
"value": 20
}
],
"pages": 5
}
}

[/code]

② sql 执行

![这里写图片描述](https://img-blog.csdn.net/20180826180051903?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhcmdlcjU=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70)

2、多表关联、分页查询接口

http://localhost/common/getAllQuestionWithStudentByPage/1/2

① json 输出

{
"code": 200,
"data": {
"total": 10,
"size": 2,
"current": 1,
"records": [
{
"id": 1,
"studentId": 3,
"name": "vv",
"content": "唐代，渝州城里，有一个性格开朗、乐观的小伙子，名叫景天。",
"date": 1534497561000,
"value": 5
},
{
"id": 2,
"studentId": 1,
"name": "cun",
"content": "雪见从小父母双亡，由爷爷唐坤抚养成人。",
"date": 1533201716000,
"value": 20
}
],
"pages": 5
}
}

[/code]

② sql 执行

![这里写图片描述](https://img-blog.csdn.net/20180826180518501?watermark/2/text/aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhcmdlcjU=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70)

六、小结

写本文的原因：

①网上有做法不合时宜的文章（自定义page类、配置版）
②官方文档使用的是配置版的，笔者采用注解版的

MyBatis 配置版	MyBatis 注解版
① 动态 sql 灵活、② xml 格式的 sql，可拓展性好	① 少一个设置，少一个错误爆发点、② 代码清晰优雅

当然，智者见智仁者见仁

参考资料：
MyBatisPlus 官方文档：分页插件：方式一、传参区分模式【推荐】

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