假设你想按br_code和月份对行进行分组，然后将日期截短到月份的开始并按其分组，然后将其格式化为字符串并在外部查询中按截短的日期排序：

SELECT br_code,
       TO_CHAR(month,'MON-YYYY') AS new_date,
       new_bill
FROM   (
  SELECT br_code,
         TRUNC(v_date, 'MM') AS month,
         min(bill_amt) AS new_bill
  from   pos_hdr
  GROUP BY
         br_code,
         TRUNC(v_date, 'MM')
UNION
  SELECT br_code,
         TRUNC(v_date, 'MM'),
         min(bill_amt)
  from   pos_prv_hdr
  GROUP BY 
         br_code,
         TRUNC(v_date, 'MM')
)
ORDER BY
       month;

或者，根据您是否希望将两个表聚合在一起（而不是将它们分开并使用UNION丢弃重复项），然后：

SELECT br_code,
       TO_CHAR(month,'MON-YYYY') AS new_date,
       MIN(bill_amt) AS new_bill
FROM   (
  SELECT br_code,
         TRUNC(v_date, 'MM') AS month,
         bill_amt
  from   pos_hdr
UNION ALL
  SELECT br_code,
         TRUNC(v_date, 'MM') AS month,
         bill_amt
  from   pos_prv_hdr
)  
GROUP BY
       br_code,
       month
ORDER BY
       month;

fiddle