MyBatis-plus实现三级联动查询、涉及分页和模糊查询的联表查询处理_mybatisplus 级联查询

作者：寸_铁 | 2024-07-15 17:35:02

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mybatisplus 级联查询

一、三级联动（以省份-城市-考场为例）

1）表设计

省份和城市存在同一张表，parentId为0代表省，parentId不为零代表对应省份下的城市。


package com.example.jiakao.pojo.entity;
 
import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;
 
import javax.persistence.*;
 
@Entity
@Data
@TableName("plate_region")
@Table(name="plate_region")
public class PlateRegionPo {
    @Id
    @TableId(type = IdType.AUTO)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column( columnDefinition = "int4" )
    private Integer id;
    @Column( columnDefinition = "varchar(64)" )
    private String name;
    @Column( columnDefinition = "varchar(64)" )
    private String plate;
    @Column( columnDefinition = "int4 default 0 " )
    private Integer parentId;
}

考场表，parentId对应城市id


import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;
 
import javax.persistence.*;
 
@Entity
@Data
@TableName("exam_place")
@Table(name="exam_place")
public class ExamPlacePo {
    @Id
    @TableId(type = IdType.AUTO)
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column( columnDefinition = "bigint" )
    private Long id;
    @Column( columnDefinition = "varchar(64)" )
    private String name;
    @Column( columnDefinition = "int4" )
    private Integer parentId;
}

2）定义返回类型vo


package com.example.jiakao.pojo.vo.list;
 
import lombok.Data;
 
import java.util.List;
 
@Data
public class ProvinceVo {
    private Long id;
    private String name;
    private String plate;
    private List<CityVo> children;
}


package com.example.jiakao.pojo.vo.list;
 
import lombok.Data;
 
import java.util.List;
 
@Data
public class CityVo {
    private Long id;
    private String name;
    private String plate;
    private List<ExamPlaceVo> children;
}


package com.example.jiakao.pojo.vo.list;
 
import lombok.Data;
 
@Data
public class ExamPlaceVo {
    private Long id;
    private String name;
}

3）mapper.xml查询语句


<?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.example.jiakao.mapper.ExamPlacePoMapper">
    <resultMap id="selectRegionExamPlaces" type="com.example.jiakao.pojo.vo.list.ProvinceVo">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <result column="plate" property="plate"></result>
        <collection property="children" ofType="com.example.jiakao.pojo.vo.list.CityVo">
            <id column="city_id" property="id"></id>
            <result column="city_name" property="name"></result>
            <result column="city_plate" property="plate"></result>
            <collection property="children" ofType="com.example.jiakao.pojo.vo.list.ExamPlaceVo">
                <id column="exam_id" property="id"></id>
                <result column="exam_name" property="name"></result>
            </collection>
        </collection>
 
    </resultMap>
    <select id="listRegionExamPlaces" resultMap="selectRegionExamPlaces">
        select
            p.id,
            p.name,
            p.plate,
            c.id city_id,
            c.name city_name,
            c.plate city_plate,
            e.id exam_id,
            e.name exam_name
        from plate_region p
            join plate_region c on c.parent_id = p.id
            join exam_place e on e.parent_id = c.id
        where p.parent_id = 0
    </select>
</mapper>

4）查询结果


{
    "code": 200,
    "message": "成功",
    "data": [
        {
            "id": 2,
            "name": "湖北",
            "plate": "鄂",
            "children": [
                {
                    "id": 3,
                    "name": "武汉",
                    "plate": "A",
                    "children": [
                        {
                            "id": 1,
                            "name": "hfgg"
                        },
                        {
                            "id": 5,
                            "name": "hgi"
                        }
                    ]
                }
            ]
        },
        {
            "id": 1,
            "name": "广东",
            "plate": "粤",
            "children": [
                {
                    "id": 7,
                    "name": "深圳",
                    "plate": "B",
                    "children": [
                        {
                            "id": 6,
                            "name": "hgiG"
                        }
                    ]
                },
                {
                    "id": 8,
                    "name": "广州",
                    "plate": "A",
                    "children": [
                        {
                            "id": 7,
                            "name": "hgU"
                        },
                        {
                            "id": 8,
                            "name": "hgUU"
                        }
                    ]
                }
            ]
        }
    ]
}

5）注意点

join、left join、right join三者的区别简单概括：join取两张表的交集，只保留有交集的条目；left join以join之前的表为主表，无论有没有交集，主表条目都存在；right join以join之后的表为主表。

二、关于带有模糊查询以及分页的联表查询处理（以考场-考试科目为例）

1）vo定义，用于保存数据时传参，与po对象字段对应，只忽略po对象中不需要用户操作的字段。


package com.example.jiakao.pojo.vo.req.save;
 
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
 
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.NotNull;
 
@Data
@ApiModel("保存考场信息的参数")
public class ExamPlaceReqVo {
    @ApiModelProperty(value = "考场id")
    private Long id;
    @ApiModelProperty(value = "考场名称", required = true)
    @NotBlank(message = "名称不能为空")
    private String name;
    @ApiModelProperty(value = "考场所属城市id", required = true)
    @NotNull(message = "所属城市不能为空")
    private Integer cityId;
    @ApiModelProperty(value = "考场所属省份id", required = true)
    @NotNull(message = "所属省份不能为空")
    private Integer provinceId;
}


package com.example.jiakao.pojo.vo.req.save;
 
import com.example.jiakao.common.validator.BoolNumber;
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
 
import javax.validation.constraints.Min;
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.NotNull;
 
@Data
@ApiModel("保存课程信息")
public class ExamCourseReqVo {
    @ApiModelProperty("课程id")
    private Long id;
    @ApiModelProperty(value = "课程名称", required = true)
    @NotBlank(message = "课程名不能为空")
    private String name;
    @ApiModelProperty(value = "课程类型",required = true)
    @NotNull(message = "课程类型不能为空")
    @BoolNumber(message = "课程类型值为0或1")
    private Short type;
    @ApiModelProperty(value = "课程价格",required = true)
    @NotNull(message = "课程价格不能为空")
    @Min(value = 0, message = "价格不能为负数")
    private Double price;
    @ApiModelProperty(value = "课程所属考场",required = true)
    @NotNull(message = "所属考场不能为空")
    private Long examPlaceId;
}

返回列表的vo定义，用于构建返回结果实体。


package com.example.jiakao.pojo.vo.list;
 
import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;
import lombok.Data;
 
@Data
@ApiModel("课程信息返回值")
public class ExamCourseVo {
    @ApiModelProperty("课程id")
    private Long id;
    @ApiModelProperty(value = "课程名称")
    private String name;
    @ApiModelProperty(value = "课程类型")
    private Short type;
    @ApiModelProperty(value = "课程价格")
    private Double price;
    @ApiModelProperty(value = "课程所属考场")
    private Long examPlaceId;
    @ApiModelProperty(value = "考场所属省份")
    private Integer provinceId;
    @ApiModelProperty(value = "考场所属城市")
    private Integer cityId;
}

2）Service和Mapper

因为我们使用的两张表中都有name字段，因此我们在构造QueryWrapper时需要明确标出时哪一张表的name字段，因此这里使用QueryWrapper而不能使用LambdaQueryWrapper。


package com.example.jiakao.service;
 
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import com.example.jiakao.common.enhance.MpPage;
import com.example.jiakao.common.enhance.MpQueryWrapper;
import com.example.jiakao.mapper.ExamCourseMapper;
import com.example.jiakao.pojo.entity.ExamCoursePo;
import com.example.jiakao.pojo.vo.json.PageVo;
import com.example.jiakao.pojo.vo.list.ExamCourseVo;
import com.example.jiakao.pojo.vo.req.query.KeyWordReqVo;
import org.springframework.stereotype.Service;
 
@Service
public class ExamCourseService extends ServiceImpl<ExamCourseMapper, ExamCoursePo> {
    public PageVo<ExamCourseVo> listCoursesPage(KeyWordReqVo query) {
        MpPage<ExamCourseVo> mpPage= new MpPage<>(query);
        MpQueryWrapper<ExamCourseVo> wrapper = new MpQueryWrapper<>();
        wrapper.likes(query.getKeyword(),"c.name");
        baseMapper.selectCourses(mpPage,wrapper);
        return mpPage.buildVo();
    }
}  }
}


package com.example.jiakao.mapper;
 
import com.baomidou.mybatisplus.core.conditions.query.QueryWrapper;
import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.baomidou.mybatisplus.core.toolkit.Constants;
import com.example.jiakao.common.enhance.MpPage;
import com.example.jiakao.pojo.entity.ExamCoursePo;
import com.example.jiakao.pojo.vo.list.ExamCourseVo;
import org.apache.ibatis.annotations.Param;
 
//官方推荐前缀Constants.WRAPPER => ew
public interface ExamCourseMapper extends BaseMapper<ExamCoursePo> {
    public MpPage<ExamCourseVo> selectCourses(MpPage<ExamCourseVo> page,
                                              @Param(Constants.WRAPPER) QueryWrapper wrapper);
}

当我们在Mapper的方法中传入和返回的类型都是IPage接口的实现时，Mybatis-plus会自动帮我们分页，也就是在查询语句后拼接limit。但是查询条件不会帮我们自动实现，需要我们手动调用QueryWrapper类型的customSqlSegment获取SQL语句然后拼接到xml中。注意使用${}进行拼接，而不是#{}，$是直接替换，而#会进行预编译。

当然我们也可以在有需要时对除BaseMapper泛型的实体以外的自定义实体构造LambdaQueryWrapper，此时我们需要增加afterPropertiesSet配置。


public class MyBatisPlusConfig implements InitializingBean {
    @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.POSTGRE_SQL));
        return interceptor;
    }
 
    /**
     * 拥有lambda cache的实体类，才能使用LambdaQueryWrapper<Entity>
     * 默认情况下只有BaseMapper<Entity>中泛型的entity才拥有lambda cache
     * 其他可以通过TableInfoHelper手动添加lambda cache
     */
    @Override
    public void afterPropertiesSet() throws Exception {
        MapperBuilderAssistant assistant = new MapperBuilderAssistant(new MybatisConfiguration(),"");
        TableInfoHelper.initTableInfo(assistant, ExamCourseVo.class);
    }
}

3）Mapper.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.example.jiakao.mapper.ExamCourseMapper">
    <resultMap id="selectCourses" type="com.example.jiakao.pojo.vo.list.ExamCourseVo">
    </resultMap>
<!--    ${wrapper.customSqlSegment} 获取自定义SQL-->
    <select id="selectCourses" resultMap="selectCourses">
        select
            c.id,
            c.name,
            c.exam_place_id,
            c.price,
            c.type,
            e.city_id,
            e.province_id
        from exam_course c
            left join exam_place e on c.exam_place_id = e.id
        ${ew.customSqlSegment}
    </select>
</mapper>

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