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需求说明：
返回值列表扩展字段值，不改变原有代码的基础上，增加返回值

编写类VO: 

public class RegionVO extends Region {//点位数量private Integer nodeCount;public Integer getNodeCount() {return nodeCount;}public void setNodeCount(Integer nodeCount) {this.nodeCount = nodeCount;}
}

 mapper.xml

<select id="selectRegionVOList" resultType="com.dkd.manage.vo.RegionVO">SELECTr.*,count( n.id ) AS node_countFROMtb_region rLEFT JOIN tb_node n ON r.id = n.region_idGROUP BYr.id<where><if test="regionName != null  and regionName != ''"> and region_name like concat('%', #{regionName}, '%')</if></where></select>

controller类：

@RestController
@RequestMapping("/manage/region")
public class RegionController extends BaseController
{@Autowiredprivate IRegionService regionService;/*** 查询区域管理列表*/@PreAuthorize("@ss.hasPermi('manage:region:list')")@GetMapping("/list")public TableDataInfo list(Region region){startPage();List<RegionVO> voList = regionService.selectRegionVOList(region);return getDataTable(voList);}
}

浏览器测试结果：

正常情况下也是可以的，但是当时测试的时候，始终缺少字段nodeCount ;

在这里我们可以换种方案：

修改mapper.xml文件

  <resultMap id="RegionVOResult" type="com.dkd.manage.vo.RegionVO"><result property="id"    column="id"    /><result property="regionName"    column="region_name"    /><result property="remark"    column="remark"    /><result property="createBy"    column="create_by"    /><result property="updateBy"    column="update_by"    /><result property="createTime"    column="create_time"    /><result property="updateTime"    column="update_time"    /><result property="nodeCount"    column="node_count"    /></resultMap><select id="selectRegionVOList" resultMap="RegionVOResult">SELECTr.*,count( n.id ) AS node_countFROMtb_region rLEFT JOIN tb_node n ON r.id = n.region_idGROUP BYr.id<where><if test="regionName != null  and regionName != ''"> and region_name like concat('%', #{regionName}, '%')</if></where></select>

再次运行，效果也是可以的！

扩展SQL： 

SELECTre.id,re.region_name,re.remark,IFNULL( tbc.node_count, 0 ) AS nodeCount,re.create_time 
FROMtb_region reLEFT JOIN ( SELECT region_id, count( 0 ) node_count FROM tb_node GROUP BY region_id ) tbc ON tbc.region_id = re.id SELECTr.*,count( n.id ) AS node_count 
FROMtb_region rLEFT JOIN tb_node n ON r.id = n.region_id 
GROUP BYr.id