（记录贴）Mybatis 数据库中用聚合函数 如何映射及赋值实体类

Posted 2021-07-22 小样5411

记录贴

找了个实习，开发中遇到一个问题记录一下，可能也是初学者会遇到的问题

SELECT basic.station_id,basic.station_name,SUM(route.road_length) AS total_kilometers,COUNT(route.id) AS total_road,COUNT(route.status=0 OR NULL) AS under_construction,COUNT(attraction.id) AS total_scenery
FROM road_road_route AS route
LEFT JOIN road_station_basic basic ON basic.route_id=route.id
LEFT JOIN road_station_attraction attraction ON attraction.route_id=basic.route_id

查询结果

可能也会有初学者跟我一样没反应过来，就是数据库中有对应实体知道怎么进行映射，但是这里用了聚合函数的字段呢，没有该字段对应的实体属性呀，这怎么弄？这里就要自己创建一个了叫XXXVo.java

这里写的字段就是数据库查出的列名对应的字段

package com.vkl.dao.vo.road;

/**
 * <p>
 * </p>
 *
 * @author yx
 * @since 2021-06-28
 */
@Data
@EqualsAndHashCode(callSuper = false)
@Accessors(chain = true)
@ApiModel(value="RoadStationBasicVo对象")
public class RoadStationBasicVo extends RoadStationBasic implements Serializable {

    private static final long serialVersionUID = 1L;

    @ApiModelProperty(value = "站点id")
    private Long stationId;

    @ApiModelProperty(value = "站点名称")
    private String stationName;

    @ApiModelProperty(value = "总公里数")
    private Long totalKilometers;

    @ApiModelProperty(value = "总道路数")
    private Long totalRoad;

    @ApiModelProperty(value = "在建公路数")
    private Long underConstruction;

    @ApiModelProperty(value = "景点数")
    private Long totalScenery;

}

然后就是Mybatis，将实体中的字段和数据库列名字段映射上

<resultMap id="BaseResultMap" type="com.vkl.dao.vo.road.RoadStationBasicVo">
     <id column="id" property="id"/>
     <result column="station_name" property="stationName"/>
     <result column="station_id" property="stationId"/>
     <result column="total_kilometers" property="totalKilometers"/>
     <result column="total_road" property="totalRoad"/>
     <result column="under_construction" property="underConstruction"/>
     <result column="total_scenery" property="totalScenery"/>
</resultMap>

然后在dao，service，controller写对应方法即可，用刚刚创建的Vo实体类作为返回

然后访问就可以看到数据了