MybatisPlus实现多表联查、分页查询_mybatisplus多表查询分页

前言

MybatisPlus对于单表的操作很方便，但是多表查询等复杂的操作还是需要在xml中写sql语句来完成。
那么，在MybatisPlus中如何实现多表联查、分页查询呢？

---

一、数据库表设计

新建学生表 student 和课程表 course
学生表

列名

注释

id

唯一标识

student_name

学生姓名

课程表

列名

注释

id

唯一标识

course_name

课程名称

student_id

学生id

二、项目目录结构

项目源码：https://github.com/dreamy-fish/mybatis-plus-resultmap

1、pom.xml

<!--lombok-->
<dependency>
    <groupId>org.projectlombok</groupId>
    <artifactId>lombok</artifactId>
    <optional>true</optional>
</dependency>
<!--mybatis-plus-->
<dependency>
    <groupId>com.baomidou</groupId>
    <artifactId>mybatis-plus-boot-starter</artifactId>
    <version>3.4.0</version>
</dependency>
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2、application.yml

mybatis-plus:
  mapper-locations: classpath:/mapper/*Mapper.xml
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3、实体类

Course

import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;

@Data
@TableName(value = "course")
public class Course {
    private Integer cid;
    private String courseName;
    private Integer studentId;
}
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Student

import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;

import java.util.List;

@Data
@TableName(value = "student")
public class Student {
    private Integer id;
    private String studentName;
    
    //一对一
    //@TableField(exist = false)
    //private Course course;
    
    //一对多
    @TableField(exist = false)
    private List<Course> courses;
}
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三、多表联查（VO对象）

1、StudentCourseVo

import lombok.Data;
import java.io.Serializable;

@Data
public class StudentCourseVo implements Serializable {

    private Integer id;
    private String studentName;//学生姓名
    private Integer cid;
    private String courseName;//课程名称
    private Integer studentId;
}
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2、StudentController

import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.dreamyfish.entity.vo.StudentCourseVo;
import com.dreamyfish.service.StudentService;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.annotation.Resource;

@RestController
@RequestMapping("student")
public class StudentController {

    @Resource
    private StudentService studentService;

    /**
     * 多表联查，一对多，分页
     * @param page 当前页
     * @param size 每页条数
     * @return
     */
    @GetMapping("pageTestB/{page}/{size}")
    public Page<StudentCourseVo> pageTestB(@PathVariable Integer page, @PathVariable Integer size){
        System.out.println("A");
        Page<StudentCourseVo> iPage = new Page<StudentCourseVo>(page, size);
        return studentService.getPageVo(iPage);
    }
}
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3、StudentService

//import省略
@Service
public class StudentService {

    @Resource
    private StudentMapper studentMapper;

    public Page<StudentCourseVo> getPageVo(Page<StudentCourseVo> iPage) {
        return studentMapper.getPageVo(iPage);
    }
}
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4、StudentMapper

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

    @Select("SELECT * from student a LEFT JOIN course b on a.id=b.student_id")
    Page<StudentCourseVo> getPageVo(Page<StudentCourseVo> iPage);
}
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5、接口测试

http://localhost:8080/student/pageTestB/1/2

---

四、多表联查（xml、一对多）

1、StudentMapper

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.dreamyfish.entity.Student;

import java.util.List;

public interface StudentMapper extends BaseMapper<Student> {

    List<Student> getAll();
}
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2、StudentService

import com.dreamyfish.entity.Student;
import com.dreamyfish.mapper.StudentMapper;
import org.springframework.stereotype.Service;

import javax.annotation.Resource;
import java.util.List;

@Service
public class StudentService {

    @Resource
    private StudentMapper studentMapper;

    public List<Student> getAll() {
        return studentMapper.getAll();
    }
}
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3、StudentController

import com.dreamyfish.entity.Student;
import com.dreamyfish.service.StudentService;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.annotation.Resource;
import java.util.List;

@RestController
@RequestMapping("student")
public class StudentController {

    @Resource
    private StudentService studentService;

    /**
     * 多表联查，一对多
     * @return
     */
    @GetMapping("testA")
    public List<Student> testA(){
        return studentService.getAll();
    }
}
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4、StudentMapper.xml

提示：如果是一对一，把collection改成assocication

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.dreamyfish.mapper.StudentMapper">

    <resultMap id="s_r" type="com.dreamyfish.entity.Student">
        <id property="id" column="id"/>
        <result property="studentName" column="student_name"/>
        <!--collection:一对多
            assocication:一对一
            -->
        <collection property="courses" ofType="com.dreamyfish.entity.Course">
            <result column="cid" property="cid" />
            <result column="course_name" property="courseName" />
            <result column="student_id" property="studentId" />
        </collection>

    </resultMap>

    <select id="getAll" resultMap="s_r">
        select * from student a left join course b on a.id=b.student_id
    </select>

</mapper>
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5、接口测试

http://localhost:8080/student/testA

---

五、多表联查（一对多、分页）

1、MybatisPlusConfig

MybatisPlus分页配置

import com.baomidou.mybatisplus.extension.plugins.PaginationInterceptor;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
@MapperScan("com.dreamyfish.mapper.*")
public class MybatisPlusConfig {

    /**
     * 分页插件，自动识别数据库类型 多租户，请参考官网【插件扩展】
     */
    @Bean
    public PaginationInterceptor paginationInterceptor(){
        return new PaginationInterceptor();
    }
}
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2、StudentController

import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.dreamyfish.entity.Student;
import com.dreamyfish.service.StudentService;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.annotation.Resource;

@RestController
@RequestMapping("student")
public class StudentController {

    @Resource
    private StudentService studentService;

    /**
     * 多表联查，一对多，分页
     * @param page 当前页
     * @param size 每页条数
     * @return
     */
    @GetMapping("pageTestA/{page}/{size}")
    public Page<Student> pageTestA(@PathVariable Integer page, @PathVariable Integer size){
        Page<Student> iPage = new Page<Student>(page, size);
        return studentService.getAll(iPage);
    }
}
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3、StudentService

//import省略
@Service
public class StudentService {

    @Resource
    private StudentMapper studentMapper;

    public Page<Student> getAll(IPage<Student> iPage) {
        return studentMapper.getAll(iPage);
    }
}
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4、StudentMapper

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

    Page<Student> getAll(IPage<Student> iPage);
}
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5、CourseMapper

//import省略
public interface CourseMapper extends BaseMapper<Course> {

    @Select("select * from course where student_id=#{studentId}")
    List<Course> selectByStudentId(Integer studentId);
}
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6、StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.dreamyfish.mapper.StudentMapper">

    <resultMap id="s_r" type="com.dreamyfish.entity.Student">
        <id property="id" column="id"/>
        <result property="studentName" column="student_name"/>
        <collection property="courses" column="id"
                    select="com.dreamyfish.mapper.CourseMapper.selectByStudentId"/>
    </resultMap>

    <select id="getAll" resultMap="s_r">
        select * from student
    </select>

</mapper>
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7、接口测试

http://localhost:8080/student/pageTestA/1/2

小结

项目源码：https://github.com/dreamy-fish/mybatis-plus-resultmap
参考：https://baomidou.com/guide/page.html