Oracle 关联group by 的表查询指定列提示不是单组分组函数

select a.*,b.*
from b_wb_jinjitask a left join 
(select caseid,wmsys.wm_concat(URL) URL from b_wb_jinjitask_upimg group by caseid) b
  on a.id = b.caseid 

提示错误，但指明需要字体获取的字段值时提示不是单组分组函数

select -- a.*,b.*
a.id,
       a.GDORDER task_code, -- 任务号
       DECODE(a.CTR_TJTIME, null, '1', decode(a.CTR_USERTIME, null, 2), '3') utask_status, -- 维保任务状态
       wmsys.wm_concat(b.URL) utask_sign_pict, -- 维保签字图片
       a.CTR_USERTIME utask_confirm_date -- 确认时间
from b_wb_jinjitask a left join 
(select caseid,wmsys.wm_concat(URL) URL from b_wb_jinjitask_upimg group by caseid) b
  on a.id = b.caseid