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">
        SELECT
        r.*,
        count( n.id ) AS node_count
        FROM
        tb_region r
        LEFT JOIN tb_node n ON r.id = n.region_id
        GROUP BY
        r.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
{
    @Autowired
    private 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">
        SELECT
        r.*,
        count( n.id ) AS node_count
        FROM
        tb_region r
        LEFT JOIN tb_node n ON r.id = n.region_id
        GROUP BY
        r.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： 

SELECT
	re.id,
	re.region_name,
	re.remark,
	IFNULL( tbc.node_count, 0 ) AS nodeCount,
	re.create_time 
FROM
	tb_region re
	LEFT JOIN ( SELECT region_id, count( 0 ) node_count FROM tb_node GROUP BY region_id ) tbc ON tbc.region_id = re.id 
	
	
	
	SELECT
	r.*,
	count( n.id ) AS node_count 
FROM
	tb_region r
	LEFT JOIN tb_node n ON r.id = n.region_id 
GROUP BY
	r.id